" GREENSTONES " OF WESTERN CORNWALL. 159 



the ground to a height of several feet, and is traversed in all direc- 

 tions by narrow veins having a similar composition ; they are, how- 

 ever, harder, and consequently the weathering of the matrix has 

 caused them to stand out in considerable relief. Below this outcrop, 

 and a few feet north-west of the foundations of the new vicarage, 

 some fresh specimens of a highly crystalline rock were obtained. 



This, which is doubtless the " greenstone " of Dr. Forbes, has the 

 usual greenish colour, is crystalline, and appears, in ordinary hand 

 specimens, to consist of an intimate mixture of a dark- coloured 

 mineral with a considerably less amount of crystalline felspar ; in 

 other localities crystals of felspar predominate. 



Although there is an almost continuous exposure of slates for a 

 considerable distance in a lane immediately east of the Tolcarn out- 

 crop, no distinctly crystalline rocks are visible ; and it consequently 

 becomes probable that they do not here extend many yards in that 

 direction, since the road runs nearly at right angles to their usual 

 line of strike. 



When thin sections of these rocks are examined under the micro- 

 scope, they are generally found to have experienced much alteration ; 

 but the extent of the change which they have undergone is far more 

 considerable in some cases than in others ; and the nature of this 

 alteration varies as well as its degree. 



Those which have undergone the least amount of metamorphism 

 are composed of crystalline felspar, augite, or diallage, magnetite, 

 titanic iron *, and occasional specks of pyrites, with a little apatite ; 

 they also occasionally contain flakes of brown mica, and, as products 

 of alteration, viridite and hornblende. The more altered rocks, 

 on the other hand, consist of a colourless semi transparent base, 

 through which hornblende and viridite are thickly disseminated. 

 In this are enclosed green pseudomorphs after augite, shadowy out- 

 lines of felspar crystals rendered to a considerable extent opaque 

 by flocculent secondary formations, crystals of magnetite, and pseudo- 

 morphs after that mineral. Some specimens also enclose crystalline 

 grains or granular patches of quartz. It will thus be observed 

 that the ultimate result of metamorphism has been the removal of 

 almost all the original minerals, and their replacement by others, 

 which nevertheless frequently admit of the forms of the primary 

 constituents being distinguished. 



It has been before stated that the nature of the successive changes 

 which have taken place varies in different localities. Thus in the 



* A large proportion of the enclosed black opaque mineral appears to have 

 the form of magnetite ; but it also occurs imperfectly crystallized, and as irre- 

 gular disseminated grains, &c. In order, therefore, to ascertain whether titano- 

 ferrite is likewise present, a small quantity of black sand was separated by a 

 careful washing of a considerable amount of pulverized rock, and the ferru- 

 ginous residue subsequently examined for titanic oxide. In this way the pre- 

 sence of that substance was clearly established ; but as the quantity is small, and 

 the exact proportion in which it occurs is of little importance to this investiga- 

 tion, its presence has been neglected in the analyses ; it is, however, probable 

 that the magnetite in these rocks is generally titaniferoiis. 



Q. J. G. S. No. 120. m 



