part 4] THE PLIOCENE DEPOSITS OF COE>"WALL. 351 



strike direction. Between the dyke and the sill is the basin- 

 shaped depression in the My lor Slates (Lower Palaeozoic) in which 

 the deposits were laid down, and thus we can explain the disposi- 

 tion of the sands in a north-easterly and south-westerly direction 

 at this jDlace. Natural sections in these deposits are rare, and 

 evidence for mapping has to be based largely on quarry- exposures, 

 on the character of the soil, and on topographic data. The arti- 

 ficial sections at present visible differ greatly from those examined 

 even 14 years ago, when the Geological Survey Memoir of the 

 district was first published ; in no case is the fossiliferous clay 

 to be seen now, and, of the three exposures available for study, 

 that of the Cornish Sand Company's pit (an enlargement of the 

 better known ' Harvey's Pit) ^ is by far the best. This pit is 

 situated immediately south of and adjacent to the road leading 

 from the village to the vicarage. The old pits west of the vicarage, 

 in which the fossils were originally obtained, are now much over- 

 grown, and all traces of the fossiliferous clays obscured. ^ 



The Cornish Sand Company's pit, which has been rapidly ex- 

 tending during the last few years, shows an excellent section 

 in the Pliocene sands. The character of these deposits varies 

 considerably both laterally'' and in depth, and apart from minor 

 irregularities in bedding, there is a marked dip of the whole series 

 of 5° north-westwards which determines the gentle slope of the 

 fields betw^een the vicarage and the village, and further explains 

 the non-appearance of the fossiliferous clay in this pit. The 

 section shows an average thickness of 20 feet of sand resting upon 

 the eroded Palaeozoic fioor, Avhich consists of slate intersected in 

 one place by an ' elvan ' dyke. The sand is overlain by 2 to 4 feet 

 of * Head ' and surface-soil. 



The late Clement Reid, writing in 1890,^ suggested that possibly 

 a similar outcrop to that of St. Erth would be ultimately discovered 

 on the watershed separating the Hayle River from the streams 

 draining into Mounts Bay at Newtown ; but careful search, both 

 here and at likely horizons throughout the whole valley from nori;h 

 to south, failed to reveal any trace of Pliocene material other than 

 that recorded by the Geological Survey as occurring at Lelant 

 Downs, half a mile north of Canons Town, 2 miles west-north-west 

 of St. Erth. In this case the evidence of outcrop, though indefinite, 

 is certainl}^ in favour of the occurrence of a small patch of Pliocene 

 deposit resting upon metamorphosed Mylor Slate at a height of 

 about 150 feet above O.D., and having much the same topo- 

 graphical aspect as the St. Erth Beds opposite. No pits have ever 

 been dug in this deposit, and the only evidence obtainable is from 

 the sandy nature of the soil and from a few blocks of ferruginous 

 sandstone found occasionally on the valley-slojDe. 



^ Mem. Geol. Surv. 1907, p. 72. 



2 At least 5 feet of sand and rubbly surface -material now filling- the pits 

 would have to be removed before exposing- the clay again, even if the thick 

 and overgrown vegetation were cleared. 



3 ' The Pliocene Deposits of Britain ' Mem. Geol. Surv. p. 60. 



