pai'fc 4] PLIOCENE DEPOSITS OF COENWALL. 349 



II. Mode or Occueeeis^ce of the Deposits. 

 {a) St. Agues. 



Flanking the nortliern part of St. Agnes Beacon, and having the 

 form of a crescent-shaped outcrop, the Pliocene deposits are here 

 resting on a belt of metamorphosed Ladock Beds (Lower Devonian) 

 sm'rounding the St. Agnes granite-mass, except in the extreme 

 south-western corner where they abut on the granite itself. The 

 height of the Beacon is 628 feet O.D., and, as we descend the 

 northern slope, the deposits are first met with approximately along 

 the line of the 420-foot contour, whence they are continuous down 

 a gentle gradient to the 35U-foot level. Both the upper and the 

 lower limits of the outcrop are concealed by several feet of 'Head,' 

 a conspicuous feature of the Quaternary geology of this region, and 

 consequently the actual contacts between the sands and the under- 

 hdng rocks are everywhere hidden. 



In addition to several minor natural exposures of the sands and 

 clays, which here, as elsewhere, constitute the principal facies of 

 these deposits, there are several overgrown pit-sections visible ; 

 while two more recent pits giving greater facilities for study occur : 

 one at Higher Bal, half a mile west of St. Agnes Church, and 

 the other at Beacon Cottage, at the foot of the Beacon on the 

 south-western side. In the former case about 12 feet of clay and 

 sand are now seen overlain by 3 to 4 feet of ' Head ' ; in the latter 

 exposure only 10 feet of grey clay are seen, the section being now 

 much obscured. The older sections mentioned by Sir Henry 

 De la Beche^ and quoted by Clement Reid^ are certainly no longer 

 in existence, and are either quite overgrown or obliterated by 

 waste-material thrown out from the adjacent mines. 



(b) St. Erth. 



On the eastern side of the Hayle-River valley, and situate about 

 half a mile from the river itself, at the village of St. Erth, is the 

 classic occurrence of Pliocene deposits, so well known for their 

 peculiar (indeed, in this country, unique) fauna. The actual 

 boundaries of the outcrop are again somewhat difficult to define 

 precisely, on account of the thick capping of ' Head ' developed. 

 Houghly elliptical in shape, about half a mile long and a quarter 

 of a mile broad, the St. Erth sands lie on a gentle valley-slope 

 between the 170- and the 50-foot contours ; the mass has a 

 discernible north-easterly and south-westerly strike, and is bounded 

 by a well-marked quartz-porphyry dyke (' Mellanear ' or ' Long- 

 Rock ' elvan) "^ on the north-west, and by an elongated sill of 

 greenstone on the south-east, both intrusions having this dominant 



' ' Report on the Geology of Cornwall, Devon, & West Somerset ' 1839, 

 pp. 258-60. 



^ ' The Pliocene Deposits of Britain ' Mem. Geol. Surv. 1890, pp. 66-67. 



^ ' The Geology of the Land's End District ' Mem. Geol. Surv. 1907, 

 pp. 63-64. 



2b2 



