158 ME. J. H. COLLINS ON SOME GEOLOGICAL [^J^y I9°9o 



The occurrence of china-clay rock beneath hard and little-altered 

 granite is by no means an uncommon phenomenon in St. Stephen's 

 and the neighbouring parishes, and it has been particularly evident 

 at Central Trevisco ; out this occurrence beneath a considerable 

 thickness of schist is, so far, unique. In the clay-pit itself, occasional 

 lenticles or irregular masses of granite in which the felspar is but 

 slightly kaolinized have been met with at various depths. One 

 such lenticle, many square fathoms in area and 5 or 6 feet thick, 

 has been recently observed near the southern side of the pit. 

 This was broken through, and china-clay rock of more than average 

 richness was found beneath it. 1 



As is almost always the case, the great clay-pit shows numerous 

 veins of secondary quartz, sometimes amethystine, or smoky or 

 rosy, and, in most instances, associated with an abundance of 

 schorl. Apart from the thick schistose overburden, there are no 

 geological features at Carpalla that cannot be seen in most of the 

 other clay-works in this part of Cornwall ; but this is certainly 

 unique, and it seems to be of vital importance in considering 

 the origin of the clay. The existence of an acre or so of fully 

 kaolinized granite lying beneath 50 or more feet of tourmaline- 

 schist is a phenomenon hitherto unknown in Cornwall, and (I 

 believe) anywhere else. 



The pneumatolytic origin of carclazyte has, it appears to me, 

 been sufficiently demonstrated now for nearly a century, and there 

 are few investigators Avho still doubt that it has been produced by 

 local acid emanations from the main masses of granite. 2 



The active agents in this pneumatolysis may in some instances 

 have included carbonic acid, as has been urged by several geologists 

 of repute, although there is very little positive evidence to support 

 the idea ; but, even so, the necessity of abundant emanations con- 

 taining chlorine, fluorine, and boron at some early stage of the 

 process is obvious enough. Lepidolite is almost always present, 

 gilhertite is frequent, and topaz is by no means rare. Fluorspar 

 occurs in considerable abundance in the china-stone quarries of 

 St. Stephen's and St. Dennis, and schorl is everywhere abundant ; 

 and so briefly we may say that fluorine and boron are always 

 present in considerable quantities. To me, therefore, the evidence 

 seems overwhelming, even more so than it did thirty years ago, 3 



1 [Since the reading of this paper, a vein of greisen containing cassiterite 

 has been found in the ' clay-rock' beneath the capping of tourmaline-schist.— 

 J. H. C, March 24th, 1901).] 



2 Those geologists who think that the West of England china-clay ' deposits ' 

 have been formed by atmospheric agencies acting from above have, it seems to 

 me, a ' hard nut to crack ' at Carpalla. For they must show how those agencies 

 have operated to great depths through thick masses of tourmaline-schist. Else- 

 where in Cornwall, as at Great Beam and Balleswidden, they have to account 

 for the operation of atmospheric agencies to depths of more than 200 fathoms 

 and far below the present sea-level. 



3 'The Hensbarrow Granite District' Truro, 1878; see also 'Nature & 

 Origin of Clays: the Composition of Kaolinite ' Min. Mag. vol. vii (1887) 

 p. 205. 



