Vol. 65 .] GEOLOGICAL FEATURES OE CAEPALLA CHINA-CLA V TIT. 155 



7. Note on some Geological Feateees observable at the Caepalla 

 China-Clay Pit in the Paeish of St. Stephen's (Cornwall). 

 By Joseph Henry Collins, F.Gr.S. (Read February 10th, 

 1909.) 



[Plate VII.] 



This clay-work is situated on the Biirngullow branch of the Great 

 Western Railway, about a mile and a half north-west of Burnguliow 

 Station, at an elevation of about 550 feet above sea-level. 



The land slopes generally from the east towards a rather deep 

 valley, the railway running from nearly south to north along the 

 eastern edge of the valley, on a low embankment across two fields 

 where the inclination is very gentle. About the middle of its 

 length, the embankment is traversed from east to west by an 

 accommodation-road, over which the line is carried by an arch of 

 masonry. East of the railway and also on the southern portion 

 of the west side, the ground has been very thoroughly explored by 

 means of pits and shafts, while the northern portion of the west 

 side is fully exposed by the great clay-excavation (the Carpalla 

 clay-pit), now over 100 feet deep. 



The surface-soil or ' meat-earth/ generally almost black and of a 

 peaty character, covers or has covered the whole estate to a depth 

 of 6 inches or more. Beneath this is everywhere a ' growan ' l or 

 stratum of subsoil, varying in depth from 2 up to 10 or 12 feet. On 

 each side of the railway from the hedge marked NX on the accom- 

 panying plan (fig. 1, p. 156), as far as the accommodation-road above 

 mentioned, this growan rests upon a stratified rock locally known 

 as kill as, which is here really a well-marked tourmaline-schist ; 

 beyond the road, it rests upon an altered granite known as china- 

 clay rock or carclazyte, and colloquially termed 'clay,' from 

 which the true china-clay or kaolin is obtained by washing. At 

 the northern end of the embankment, the clay-rock passes under a 

 rather soft variety of granite, the felspar of which has not been 

 sufficiently kaolinized to be of value to the clay-worker. This soft 

 granite is also capped by a layer of growan. 



The accommodation-road already mentioned passes along a line 

 of fault which has let down the ground to the south for some 

 fathoms. There is no indication of this fault at the surface other 

 than a series of springs ; but very slight excavation suffices to show 

 the difference of the rocks on either side of the springs. The course 

 of this fault is roughly east and west, and its hade or underlie about 

 30 degrees from the vertical towards the south. Nothing is known 

 of its extension into the hill eastwards ; in the opposite direction it 

 soon splits up, and is lost in the decomposed granite of the valley ; 



1 Growan is the local term for a disintegrated subsoil which is mainly of 

 granitic origin. 



