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Read at the Annual Conference of the Ussher Society, January 1998

GEOCHEMICAL FINGERPRINTING OF WEST CORNISH GREENSTONES AS AN AID TO PROVENANCING NEOLITHIC AXES

M. MARKHAM AND P. A. FLOYD

M. Markham and P.A. Floyd. 1998.

Geochemical fingerprinting of west Cornish greenstones as an aid to provenancing Neolithic axes.

Geoscience in south-west England

Of the large number of Neolithic stone axes made of greenstone, some 392 (referred to as Group 1 axes) are believed to have been manufactured in west Cornwall around the Mount's Bay area.

To aid the location of the greenstone that provided the materials for the axes, geochemical fingerprinting of the axes and greenstone outcrops was undertaken in the Mount's Bay area to both discriminate the greenstone localities and provide a basis for matching Group 1 axes.

 Non-destructive analysis of the axes was determined by a portable XRF Spectrometer unit that gave comparable results for selected greenstone samples to standard laboratory-based XRF techniques. Geochemical fingerprinting of the greenstone localities by portable XRF spectrometer provided a degree of discrimination between them, although preliminary data on the axes suggests that there is not a very strong correlation between axe composition and possible greenstone sites.

 Further work is required on other greenstone localities and axes as current data does not conclusively point to an origin in this area of west Cornwall.

M. Markham, Department of Earth Sciences, Open University, Walton Hall, Milton Keynes, MK7 6AA P. A. Floyd, Department of Earth Sciences, Keele University

The Devonian and Lower Carboniferous sequences of southwest England are characterised by episodes of volcanic activity with the production of largely submarine pillow lavas and high-level intrusive sills and sheets .

 During the Variscan orogeny the rocks were subjected to low-grade alteration and the volcanics developed secondary assemblages indicative of the pumpellyite and greenschist facies of regional metamorphism (Floyd, 1995).

Subsequently, on the intrusion of the granite plutons, contact metamorphic effects were locally superimposed with the production of new assemblages and textures. Overall, the effects of alteration on the predominantly basic volcanic rocks was two fold: (a) variably developed secondary assemblages superimposed on a still recognisable primary basic assemblage (pyroxene-plagioclase-ilmenite±olivine), and (b) the textural effects of deformation with the progressive development of a foliation and eventually a marked schistosity.

From the industrial viewpoint the mineralogical alteration effects on the basic volcanics has strengthened them and many of the massive (non-foliated) intrusive basic rocks are used for roadstone (Edmonds et al., 1969). Stone Age man also appreciated the durable qualities of the altered basic rocks and fashioned many hand axes from what is generally referred to as "greenstone", that is, low-grade metamorphosed dolerites and basalts.

STONE AGE AXES

Stone axes are grouped according to petrological, mineralogical and textural criteria. Over 40 different axe groups are recognised from a total of 7625 axes found in Britain (of which only 3546 have been grouped) (Clough and Cummings, 1988). Just over 1000 axes are made of greenstone, of which 392 members (referred to a "Group 1") have been identified as being manufactured in west Cornwall and possibly originating from the Mount's Bay area (Figure 1), largely on the basis of the site of origin and the local presence of greenstone (Stone, 1951). Group 1 axes, originally catalogued in the 1940s (Keiller, 1941), are broadly described as uralitized gabbro with original pyroxene and feldspar, with the characteristic development of a uralitic fringe of blue-green amphibole around the primary pyroxene; epidote, sphene and chlorite are common accessory minerals. Because this assemblage and texture is a very common feature of many Cornish greenstones, the actual outcrops, representing the stone age factories used by Neolithic man, have never been positively identified. However, the assumed manufacture from this area is still largely based on the apparent similarity between a petrographic examination of thin sections of greenstone axes and the outcrops in the Mount's Bay area. To date no definite match has been achieved, such that archaeologists have suggested that the outcrops are actually underwater and have yet to be sampled! (Evans, 1962)

PROJECT OBJECTIVES AND APPROACH

The prime object of this work is to determine from where the Group 1 greenstone axes originated, using a combination of petrographic and textural data coupled with new geochemical discrimination techniques. Petrographic comparisons between axe and potential outcrop are a standard method to identify sources (Markham, 1997) although in this case the results have proved inconclusive. Geochemical fingerprinting of the axes, especially trace element distributions, was considered a more sensitive and selective approach. Also, there already exists a broad-based set of geochemical data on Cornish greenstones (both intrusives and lavas) that shows some significant chemical distinctions between different localities composed of broadly similar mineralogical assemblages (Floyd, 1995). If both axes and greenstone outcrops can be sufficiently discriminated geochemically this provides a sound basis for comparison and the strong possibility of linking axe to source.

However, in order to provenance axes by geochemical means, a non-destructive analytical method is needed to preserve the axe, although ideally should be a similar technique to that used to analyse field samples (to minimise inter-technique errors). In this connection we have used X-ray Fluorescence Spectrometry, both a portable instrument (PXRF) and a laboratory-based instrument (LXRF)(see below).

ANALYTICAL METHODS AND COMPARISONS

XRF Spectrometry determines the concentration of elements in a sample by measuring the variable intensity of fluorescent energies given off by a sample when illuminated by x-rays.

 

Figure 1

Map of west Cornwall showing the occurrence of greenstone outcrops sampledfor this study. (Greenstones -solid ornament, Granite ‘+ 'ornament)

Considerable preparation is required prior to analysis, involving crushing to a fine powder and subsequent pelletizing, and thus destroying the original sample. This is standard procedure for laboratory-based instruments, but is not an option for valuable and irreplaceable axes. In this context the PXRF has a number of advantages - it is portable (carried by one person), is nondestructive, the samples need no preparation, and can work off mains or battery. Provided the sample has a reasonably planer surface greater than 25 mm in diameter it can be geochemically analysed. For PXRF the radiation energies needed to fluoresce the sample are obtained through the radioactive decay of isotopes of Fe, Cd and Am. The reflected energies are detected by a high purity HgI2 detector which requires no external cooling. Table 1 summarises the main features of both PXRF and laboratory XRF (LXRF) equipment

 

Figure 2

Envelopes enclose geochemical data from two major greenstone outcrops around Penlee Point and adjacent foreshore, and the Carn Gwavas quarry, derived by PXRF (MM(PXRF)), laboratory-based XRF by Markham (MM(LXRF)) and by Floyd (PAF(LXRF)). Note the general overlap ofPXRF and LXRF data from the two localities.

Before meaningful geochemical comparisons can be made between the axes and potential sample localities, it is necessary to check that the PXRF instrument is capable of producing similar results to laboratory based XRF instruments (at Open University) and also existing data from the literature derived by similar means (at Keele University). A selection of greenstone samples collected from around Penlee Point (Penlee Lifeboat station and Mousehole foreshore) and from within and shore side of the Penlee Quarry at Cam Gwavas were analysed by LXRF and PXRF at the Open University and compared with existing data by Floyd (1976, 1983, 1984) and Floyd and Al-Samman (1980). The results for the three sets of data (PXRF, LXRF, literature) are shown in Figure 2 and Table 3, and clearly demonstrate that each data source can separately define and discriminate the two sampled sites (foreshore versus quarry). We consider that the overlap of geochemical data derived from the PXRF is sufficiently good to allow discrimination of the axes and sources, and also compares well with existing LXRF-derived data.

FEATURES OF GREENSTONE SITES

Figure 1 shows the sites subjected to preliminary evaluation as potential axe sources. Initially the choice of locations was determined by the proximity to Mount's Bay and environs (the suggested origin of Group 1 axes), and the amount of existing data available. Table 2 summaries the location and nature of emplacement for the greenstone bodies; detailed descriptions of some of the following sites may be found in Floyd et al. (1993).

Cudden Point [SW 548 275] - This is a relatively coarse, massive, sheet-like intrusive body of greenstone composed of meta-gabbro and

meta-dolerite, with strongly foliated margins (Floyd and Lees, 1972). It shows the typical development of pale green uralitic actinolite fringing large clinopyroxene prisms, some of which may exhibit pigeonite lamellae. Petrographically it is characterised by rare crystals of brown primary amphibole, two generations of secondary amphibole (pale uralite and blue-green), and replaced ovoids of olivines surrounded by pyroxene.

Trenow [SW 529 303] - This site covers a series of scattered outcrops around Trenow Cove including a small quarry at Perranuthnoe. Periglacial "head" covers much of the area behind the wave-cut platforms and it is not clear if more than one massive intrusive body exists here. The greenstones here are similar to Cudden Point, but generally finer grained and composed of metadolerite and meta-basalt. The metamorphic mineralogical growth has developed further with a higher proportion of secondary minerals, especially amphibole, albite, epidote, chlorite and sphene. In order to decrease the potential variability of results all data presented below comes from the west end of Trenow Cove.

Penlee Point [SW 474 269] - This includes much of the Mousehole foreshore around the Penlee Life Boat station to Carn Gwavas to the north. It includes the two sills found in the inland quarries at Penlee Point proper. Although within the Land's End granite aureole, the greenstones here show similar mineralogical and textural features to low-grade regional meta-basites elsewhere in west Cornwall. They are mainly meta-dolerites with uralitic actinolite, grading to actinolite-plagioclase hornfelses with a weak foliation. Evidence of contact metasomatism is shown by the development of rare blue zoned tourmaline, and the patchy hydrothermal "bleaching" of some outcrops.

Carn Gwavas [SW 470 280]- This is restricted to the huge (now disused) quarry between the shore and the granite inland and the immediate foreshore east of the quarry. Although often considered and quarried as "greenstone", this rock type is not basic, but more intermediate in composition and thus quite distinct mineralogically and chemically from typical greenstones. It has an interlocking granular texture composed almost entirely of albite and ragged amphibole. It displays variable degrees of hydrothermal alteration with the development of kaolinite, biotite, chlorite, as well as a complex of sulphide mineralization veinlets (Floyd, 1965). Axes derived from this source would be very distinctive mineralogically and chemically.

Zennor [SW 450 394] - This forms a small resistant headland of greenstone west of St. Ives within the granite contact aureole. It is mainly a fine-grained amphibole-rich hornfels of massive aspect. Gurnard's Head [SW 432 387] - This headland is largely composed of a massive sheet-like body that grades upwards into a pillow lava sequence and represents a high-level intrusion near the sediment-water interface. Again within the granite aureole, the original mineralogy and texture have been replaced by a hornfelsic matte of actinolite with subsidiary plagioclase and rare biotite replacing amphibole. Chlorite-filled amygdales in the pillowed section are still recognisable. Table 2 shows the mineralogical features of the greenstones found at these sites, as well as summarising some of their geochemical characteristics.

GEOCHEMICAL DISCRIMINATION OF GREENSTONE SITES

In this section we attempt to discriminate the different selected sites on the basis of PXRF data alone prior to any comparison with axe data derived by the same technique. The PXRF has been set up to measure 13 elements such that a large number of binary plots can be generated. In this preliminary exercise, however, a number of diagrams plotting absolute abundances measured in ppm have been selected to illustrate the chemical diversity of the greenstone sites and provide a measure of discrimination (Figure 3 and Table 4). We have chosen to present the data as averages ± 1sd since this presents a clearer indication of the outcrops and reduces clutter on the plots. 2SD 'space' will be used in the final assessment although it is recognised that a multivariate statistical approach will be needed in the future.

As seen from the plots in Figure 3, a number of chemical features characterise the sites which can be summarised below:

(a) In all plots, the Cam Gwavas body is geochemically distinctive relative to the rest, being typified by high Zr and Y, coupled with low Ti, Fe, and Sr. Also, as mentioned above, it is also mineralogically and texturally distinctive. These features provide a good discrimination for this type of "greenstone", whereas some of the other greenstone locations have more overlapping characteristics.

(b) Cudden Point also has a distinctive chemistry relative to the other greenstones, with very low incompatible element contents, especially Ti, Zr, Y and Nb; the Zr/Y ratio is also low at between 2-6.

(c) The Trenow data shows the largest standard deviation of data often overlapping other greenstone sites, with the exception of Cudden Point and Cam Gwavas. There is a degree of chemical overlap between Penlee Point, although this locality is within the granite aureole and would be expected to show some mineralogical and textural differences.

(d) There are small chemical differences between the actinolite-bearing greenstones (hornfelses) within the granite aureole at Gurnards Head and Zennor. In general, Gurnards Head has systematically higher Fe, Y and Ti than Zennor.

As seen (Figure 3) discrimination is by no means clear cut for all greenstone compositions. The log plot of Ti versus Zr provides one of the better discriminations with different locations showing partial separation on a curved trend, starting with (i) Cudden Point, (ii) Zennor, (iii) Gurnards Head + Penlee, (iv) Trenow and finally (v) Carn Gwavas. The log plot of Y versus Zr, on the other hand, has an almost linear trend with the same progression.

 Portable XRF Xd XRF  

Precision (relative error) usually 10% or better Major elements to 0.5% Minor elements to 1 - 5 %  

  (of value measured)  

Operation One person portable, hand held or stand mounted sensor, battery or mains powered Laboratory based, high power x-ray tube and cooling required  

Detection limit Major elements to 100ppm Trace elements to 3 ppm Trace (rarely <1 -2 ppm)  

Measurement time Typically 3- 5 minutes depending on age of sources 5 -40 minutes depending on precision required  

Sample Preparation None, other than presenting a clean, dust free surface Pressed powder discs Glass pellets  

Cost Machine £50,000 Loan £1,000 per week £120,000  

Range of Elements Heavier than potassium Heavier than boron  

Table 1. Comparison ofportable and laboratory based XRF equipment (after Markham, 1997: Potts, 1995)  

  

PRELIMINARY AXE DATA

 The axe average and range is plotted in Figure 4 (with data in Table 4) and compared with the geochemical fingerprints for the greenstone sites. The Ti versus Zr plot chosen to illustrate the comparison shows the axe range is comparable with both Penlee and Gurnards Head, with Carn Gwavas, Cudden and Zennor outside the axe range. Whilst encouraging, these are early results and more work will be required to build up a conclusive picture.

CONCLUSIONS

One of the most important conclusions to draw from this type of geochemical fingerprinting of archaeological artefacts is that a PXRF spectrometer can provide sensitive and accurate results without the destruction of the specimen. We have demonstrated that comparable data can be obtained from a PXRF instrument relative to a standard laboratory-based XRF spectrometer and without the problem of sample

  

Figure 3

Geochemical data derived by PXRF data (averages ± 1 sd) from selected greenstone localities in west Cornwall. Note that some diagrams provide a degree of discrimination between selected greenstone localities.  

At the moment only a preliminary assessment and comparison between the greenstone sites and the axes can be made. This is a consequence of discovering that the geochemistry of the Group 1 axes was more heterogeneous than expected with some axes having compositions that lay well outside the normal range. Histogram plots of axe elements showed a non-standard distribution and upon investigation some of the previously classified axes may have to be regrouped. Further work on existing petrographic thin sections and accession records is being carried out and this will need to be concluded before axes are eliminated from Group I. Hence this feature suggests that all the so-called Group 1 axes may not belong to this group or indeed be derived from Cornish greenstones at all. Thus, some 50 Group 1 axes have been measured by PXRF and an average together with a characteristic range calculated. The range identified in Table 4 has been taken from the aforementioned frequency histograms of the axe data and does not represent a standard deviation for the full data set, that is, it excludes those axes with apparently non-Group 1 chemistry.  

Outcrop Geological location Type of Intrusion Mineralogy Texture Geochemistry

Cudden Point Outside L.E. aureole, but affected by Godolphin granite Major sill Primary: olivine - clinopyroxene -plagioclase - opaques, rare primary amphibole

Secondary: uralite & blue green amphibole, sphene, albite ophitic - meta gabbroic/ meta doleritic - foliated margins high Cr & Ni

low incompatible (Y,Zr,Ti, etc,.)

Trenow Cove As above sheet - like intrusion Primary: clinopyroxene - plagioclase -opaques

Secondary: actinolite - albite - epidote -chlorite - sphene metadolerite - sub ophitic texture of uralitic actinolite less primitive than Cudden, more fractionated

Carn Gwavas Within L.E. aureole <600 m from granite outcrop pipe - like intrusion Primary: None remaining Secondary: albite - biotite - chlorite -sphene - common sulphide mineralisation broadly granular, result of recrystallisation through contact metamorphism alkaline, with high incompatibles (esp Ti,Nb,Y & Zr)

Penlee Within L.E. aureole <800 m from granite outcrop small sills Primary: clinopyroxene - opaques Secondary: uralitic actinolite - albite - rare tourmaline metadolerite - sub ophitic to foliated at edge of intrusion typical of intraplate basalts (OIB), with wide variation due to fractional crystallisation

Gurnards

Head Within L.E. aurole < 1km from granite outcrop pillow lavas & high level intrusions Primary: None remaining (probably pyroxene - plagioclase - ilmenite). Secondary: actinolite - plagioclase -opaques - biotite, subsequent alteration to chlorite meta-dolerite with hornfelsic texture. tholeiitic affinities with enriched E- MORE features

Zennor Within L.E. aureole < 1 km from granite outcrop unknown - but probably sill like Primary: None remaining

Secondary: actinolite - plagioclase - opaques,

plus chlorite meta-dolerite no detailed geochem available,

Table 2. Summary of geochemical and textural features (with additional material from Floyd (1993))

Sample Reference | | Source | Location | Fe (ppm) | Ti (PPm) | Zr (ppm) | Y (ppm)

KPEN01#1V PXRF Markham Carn Gwavas 35626 2489 1948.93 110.56

KPEN01#2V PXRF Markham Carn Gwavas 28833 2830 1177.15 100.83

KPEN02#1V PXRF Markham Carn Gwavas 44346 4479 1127.14 95.12

KPEN02#2V PXRF Markham Carn Gwavas 41888 4100 1095.35 106.50

KPEN1A#1V PXRF Markham Carn Gwavas 22900 3170 1019.09 126.05

KPEN1A#2V PXRF Markham Carn Gwavas 37114 3479 1037.76 74.85

KPEN2A#1V PXRF Markham Carn Gwavas 34742 4530 1203.46 89.89

KPEN2A#2V PXRF Markham Carn Gwavas 28507 3524 1103.32 81.36

LE10 LXRF Floyd Carn Gwavas 63644 3657 1575.00 127.00

LE11 LXRF Floyd Carn Gwavas 64468 3357 1260.00 132.00

LE6 LXRF Flovd Carn Gwavas 59797 3957 850.00 150.00

LE7 LXRF Floyd Carn Gwavas 66690 4436 780.00 185.00

M27 LXRF Floyd Carn Gwavas 38634 3777 775.00 147.00

M62 LXRF Floyd Carn Gwavas 23255 4616 1085.00 126.00

M63 LXRF Floyd Carn Gwavas 59331 4616 1010.00 117.00

PEN001 LXRF Markham Carn Gwavas 36231 3459 1280.00 123.80

PEN002A LXRF Markham Carn Gwavas 39029 3915 1110.00 99.40

PEN002 LXRF Markham Carn Gwavas 46723 3879 1028.00 118.20

KPEN04#1V PXRF Markham Mousehole 77431 13478 170.88 22.82

KPEN04#2V PXRF Markham Mousehole 76743 14605 180.26 22.76

M35/3 LXRF Floyd Mousehole 91099 13668 98.00 20.00

M37/2 LXRF Floyd Mousehole 96283 11450 140.00 28.00

M38 LXRF Floyd Mousehole 79144 11150 133.00 26.00

PEN004 LXRF Markham Mousehole 80996 13824 162.00 25.80

KPEN03#1V PXRF Markham Penlee Point 62547 11518 296.58 31.95

KPEN03#2V PXRF Markham Penlee Point 86293 11244 153.10 29.64

KPEN03#3V PXRF Markham Penlee Point 78401 20440 274.85 35.66

KPEN03#4V PXRF Markham Penlee Point 83990 19247 168.88 25.79

LE12 LXRF Floyd Penlee Point 77542 19543 203.00 37.00

M39 LXRF Floyd Penlee Point 93439 24219 208.00 43.00

M45/2 LXRF Floyd Penlee Point 110903 16006 137.00 27.00

M55 LXRF Floyd Penlee Point 79991 7254 92.00 16.00

M71 LXRF Floyd Penlee Point 93789 15287 128.00 29.00

PEN003 LXRF Markham Penlee Point 88830 20005 260.00 42.30

Table 3. Samples analysed by portable XRF (PXRF) and laboratory XRF (LXRF) in order to assess similarity of process.

Location Number of Samples Number of Readings Fe  TI  Sr  Y  Zr  Nb  

   ppm SD ppm SD ppm SD ppm SD ppm SD ppm SD

Carn Gwavas 4 8 34244.49 7189.14 3575.07 747.76 91.47 9.13 98.15 16.52 1214.03 303.44 86.88 6.82

Cudden Point 4 9 76919.88 7622.18 7209.62 3113.32 691.08 482.36 10.09 5.95 61.26 14.00 4.99 2.88

Gumards Head 4 8 85791.26 2239.18 15523.64 1619.35 272.28 53.85 35.07 4.89 157.17 19.17 8.85 2.56

Penlee Point 2 6 77567.47 8299.36 15088.50 3906.26 382.76 52.22 28.10 5.22 207.42 61.65 33.85 12.48

Trenow Cove 5 14 69485.64 11552.08 9345.47 1918.02 671.02 354.91 28.49 7.82 357.46 197.31 43.42 23.73

Zennor Point 3 7 68293.94 3987.44 9365.85 624.37 216.02 40.90 17.57 3.52 116.93 10.27 6.99 3.47

Group I Axes 50 118   14026.00 4421.00     184.60 37.60   

Table 4. Summarised data used in Figures 3 and 4.

Notes: One reading per sample, except where annotated '#nV', etc, for PXRF data, where n represents the n th reading for that sample.

| xCam Gwavas » Cudden o Gurnards t-tead + PanlM Point a Trenow Cove oZannorPoint 1 Axe Average

10000 T

.1 ml ,i HH

—-i—

10-1-1--—I

1000 10000 100000 TI (ppm)

Figure4

Ti-Zr plot with average and range of geochemical data for Group 1 axes superimposed on the fields for greenstones derived from various localities in west Cornwall. Note that the mean overlaps Penlee and Gurnards Head, but is separate from other localities.

preparation. In this context the PXRF data was used to generate a chemical data-base from a number of greenstone localities in west Cornwall and also analyse a selection of Neolithic axes (referred to as Group 1 axes) thought to have been manufactured from greenstone material in the Mount's Bay area.

A selection of major and trace element binary plots allows a degree of selective discrimination of the different greenstone localities. Different plots highlight the specific chemical features of the greenstones, although there is often considerable overlap between some localities. More work needs to be carried out to identify the best set of geochemical discriminants. At the present time the Ti-Zr diagram provides the best discrimination, but this must be used in conjunction with other specific characteristics, such as high or low abundances of Ni, Cr, Fe and Y.

Finally the average and range of Group 1 axes plotted on the Ti-Zr diagram reveals an encouraging overlap with one Mount's Bay site (Penlee) and only partial overlap with the other greenstone locality data. The hypothesis that Mount's Bay was the source of Group I axes remains unproven, but these results and ongoing work have a good chance of resolving the 50 year old issue.

ACKNOWLEDGEMENTS

We would like to thank Peter Webb and Olwen Williams-Thorpe for valuable comments on early drafts of this paper and to The Ian Gass Fund of the Open University for assistance with the provision of the PXRF. REFERENCES

CLOUGH T.H.McK., CUMMINGS W.A. 1988. Stone Axe Studies, CBA Research Report 67

EDMONDS, E.A., WRIGHT, J.E. and WILLIAMS, M. 1969. British regional geology: South-West England, 3rd. ed. Institute of Geological Sciences, HMSO, London.

EVANS, E.D., GRINSELL, L.V., PIGGOT, S. and WALLIS, F.S. 1962. Fourth report of the sub-committee of the South Western Group of Museums and Art Galleries on the petrological identification of stone axes. The

Prehistoric Society 10, 209-266

FLOYD, P.A. 1965. Argillization of basic homfelses from the Land's End granite aureole, Cornwall. Clay Minerals, 6, 45-58.

FLOYD, P.A. 1976. Geochemical variation in the greenstones of SW England.

Journal of Petrology, 17, 522-545.

FLOYD, P.A. 1983. Composition and petrogenesis of the Lizard Complex and pre-orogenic basaltic rocks in south-west England. In: The Variscan Fold belt in the British Isles, Ed: P.L. Hancock. Adam Hilger, Bristol, 130-152.

FLOYD, P.A. 1984. Geochemical characteristics and comparison of the basic rocks of the Lizard Complex and the basaltic lavas within the Hercynian troughs of SW England. Journal of the Geological Society of London, 141, 6170.

FLOYD, P.A. 1995. Igneous activity- basaltic volcanism in the Rhenohercynian Zone, N. Europe. In: Pre-Permian geology of Central and eastern Europe. Eds: Dallmeyer, RD., Franke, W. and Weber, K. Springer-Verlag, Berlin, 59-81.

FLOYD, P.A. and AL-SAMMAN, A.H. 1980. Primary and secondary chemical variation exhibited by some west Cornish volcanic rocks.

Proceedings of the Ussher Society, 5, 68-75.

FLOYD, P.A. and LEES, G.J. 1972. Preliminary petrological and geochemical data on the Cudden Point greenstone. Proceedings of the Ussher Society, 2, 421-423.

FLOYD, P.A., EXLEY, C.S. and STYLES, M.T. 1993. Igneous rocks of SW England. Geological Conservation Review Series no.5, Chapman and Hall, London, 256pp.

KEILLER, A., PIGGOT, S. and WALLIS, F.S. 1941. First report of the subcommittee of the South Western Group of Museums and Art Galleries on the petrological identification of stone axes. The Prehistoric Society 2, 1273-1277

MARKHAM, M, 1997, Geology and Archaeology: a search for the source rock used by British Neolithic Axe Makers. Open University Geological Society, 25 Anniversary Edition.

POTTS, P.J., WEBB, P.C. and WILLIAMS-THORPE, O. 1995. Analysis of silicate rocks using field portable x-ray flourescence instrumentation incorporating a mercury (II) iodide detector: A preliminary assessment of performance. Analyst 120, 1273 - 1278

STONE, J.F.S and WALLIS, F.S. 1951. Third report of the sub-committee of the South Western Group of Museums and Art Galleries on the petrological identification of stone axes. The Prehistoric Society 112-113

223

“1 mile SE of Callington on the A388,

from the S side of which a lane leads to Westcott. Site lies on hillside to N. First recognised as a Neolithic henge in 1951, the somewhat ploughed down bank and internal ditch cover an area 90m in diameter. An indistinct entrance occurs on the south side, but there appears to be no corresponding causeway over the ditch. There is no visible evidence to suggest that there were any internal stone or timber settings. Like Cornwall’s other two surviving henges...this site was positioned close to a major prehistoric trackway; trade may have played a part in the function of the henge. Balstone Down, close to the north, was the source of greenstone for the manufacture of Neolithic axes. The bank of the henge reaches a height of 1.0m; the maximum depth of the ditch is 1.2m.