28 CORNISH POST-TERTIARY GEOLOGY. 



ciently definite to enable one to make a rough correlation with the 

 beds given in Mr. Colenso's section' of the Happy Union Works, 

 Pentuan, where a bed of sea sand with shells, 4 inches thick, occurs 

 between beds containing forest remains, at 42 feet from the surface ; 

 but in the uppermost bed, 20 feet thick, sea sand is intercalated with 

 river sediments ; and a row of wooden piles was met with, their tops 

 being 24 feet from the surface, and on a level with spring-tide low- 

 water mark. Mr. Henwood ^ says that the tin ground in Pentuan 

 Works, near the sea, is below the sea-level and covered by sea sand 

 and shells, whilst in the stream works higher up the valley, the tin 

 ground rests on granite, and is covered by recent alluvia. 



Mr. Rashleigh described^ the stream works of Poth and Sandry- 

 cock, in the valley between St. Austell and St. Blazey. The vale has 

 so gentle a slope that, but for a flood hatch, the salt water would flow 

 into Poth. It opens into the Par Estuary, the level of its basin being 

 considerably below low-water mark. The accumulation of a beach 

 bar prevents the sea from going up the moor, except by the adit. 



Section of Stream Works at Poth, near the sea, and Sandrycock, 

 near the middle of the vale. 



1. Vegetable mould about 



2. Gravel and micaceous sand, mixed with loam in alternate beds 



3. Light-coloured clay with a little mica, with traces of decaying 



roots 



4. Black peat 



6. Light-coloured clay 



6. Stiff light-brown clay, with light-bluish spots, containing de- 



cayed vegetable matter , 



7. Sea sand and clay mixed 



8. Very fine micaceous sea sand, with comminuted shells and bits 



of slate 



9. Coarser sand without shells 



10. Solid black fen, with few vegetable remains (not used for fuel) 



11. Tin ground and loose stones of various kinds 



12. Killas, on which tin ground and in some places yellow clay rests 



At Poth, near the bottom of the sea sand, and upon the sea mud, 

 horns of deer and wild oxen were found. A pair of the latter 

 measured 15 inches in circumference at the base. 



1 lUd. pp. 401, 402, 403. 



- Trans. Eoy. Geol. Soc. Corn. vol. v. p. 129. 



3 Ibid. Tol. ii. pp. 282, 284, 286. 



