

OF CENTRAL AND EASTERN CORNWALL. 495 



a dolerite from the same neighbourhood, it will be observed that 

 they differ chiefly in the amount of silica, alumina, and mag- 

 nesia which they respectively contain. Both in hand-specimens 

 and in thin sections all traces of definite crystalline structure have 

 entirely disappeared ; but the various progressive changes observed 

 during the examination of an extensive series of these greenstones 

 render the igneous origin of this foliated rock by no means im- 

 probable. 



The slaty blue elvans found between St. Erth and St. Stephens 

 (in Bramwell) have a chemical composition identical with that of the 

 altered dolerites, and may be, as was formerly suggested, highly 

 metamorphosed ash-beds. They occasionally, however, appear to 

 exhibit indications of having been to some extent vesicular ; and 

 recent investigations would perhaps point to the greater probability 

 of their being sometimes true, although much-altered, igneous rocks. 

 The highly basic crystalline slaty hornblendic rocks of the neigh- 

 bourhood of Penzance sometimes afford traces of former vesicularity ; 

 but if, as has been suggested by Mr. Allport, these are likewise of 

 eruptive origin, they must, if originally of the same composition as 

 the neighbouring dolerites, have lost ten per cent, of their silica, and 

 have assimilated a corresponding amount of iron oxides. 



De la Beche * regarded such rocks as highly altered ash-beds ; 

 but it is not improbable that some of them may have originally been 

 flows of volcanic mud. 



No rocks of this character were met with either in Central or in 

 Eastern Cornwall. 



There can be no doubt, however, that, under certain conditions, 

 rocks of igneous origin acquire a degree of schistosity which is often 

 exceedingly puzzling to the geologist, as it is sometimes impossible to 

 determine where foliated traps cease, and where metamorphosed slates 

 begin. It is nevertheless certain that many of the foliated rocks 

 intercalated among the greenstones of Western Cornwall must 

 belong to the category of highly altered sedimentary deposits. 



The felspar in the slaty agglomerates is almost entirely plagioclase, 

 and is derived from the disintegration of greenstones, and not from 

 that of granite. 



The age of these rocks may, for the following reasons, be regarded 

 as being generally greater than that of the granite. 



First, The vesicular lavas, as well as many of the slaty horn- 

 blendic bands, are evidently contemporaneous with the slates among 

 which they are bedded, while the latter are often either displaced by 

 the granite, or traversed by granitic protrusions in the form of 

 veins. 



Secondly. The eruptive dolerites which break through the sedi- 

 mentary beds do not traverse the granite ; but, on the contrary, they 

 are frequently displaced by the latter rock. 



The relative ages of the more important Cornish rocks were 

 clearly pointed out in 1839 t by the late Sir H. T. De la Beche, 



* Geological Observer, p. 702. 



t Geology of Cornwall, Devon, and West Somerset, p. 165. 



Q. J. G. S. No. 1.35. 2 l , 



