FRAGMENTS OP OTHEE EOCKS CONTAIITED IN GEANITE. 5 



of thin sections ; and I have consequently been induced to avail 

 myseK to some extent of those methods of research in the hope of 

 being thereby enabled to more clearly determine than has hitherto 

 been done the nature and origin of such bodies. In carrying out 

 tliis work, I first examined some of the principal quarries in the 

 granite districts of the west of England, and subsequently those in 

 the neighbourhood of Shap, in Westmoreland. I afterwards made 

 myself acquainted with those in the vicinity of Aberdeen and Peter- 

 head in Scotland, and examined numerous specimens from the dis- 

 trict about Fort William collected by Mr. C. W. Merrifield, F.E.S. ; 

 finally the granite- quarries situated in the vicinity of Newry and 

 Castlewellan in Ireland were visited. 



In the following pages I propose to describe the various granites 

 and their respective inclusions in the order in which the several dis- 

 tricts were examined, beginning with a quarry at Lamorna Cove in 

 "West Cornwall, five miles south-west from the town of Penzance. 



English Granites, — The granite-quarries at Lamorna are situated 

 mainly in the cliff forming the more easterly shore of Lamorna Cove, 

 and they have, at various times, been somewhat extensively worked 

 for building-material. The granite from this locality is grey in 

 colour and moderately coarse in structure, occasionally enclosing 

 crystals of white felspar an inch and a half in length, together with 

 numerous smaller ones possessing a distinctly greenish shade. 

 Crj'stalline granules of a brownish transparent quartz are also abun- 

 dant, as is likewise a nearly black mica, which is disseminated in 

 minute scales throughout the other constituents. Black patches are 

 exceedingly abundant in this granite, and have generally the 

 appearance of fine-grained enclosures with irregular outlines ; not 

 unfrequently these exhibit a texture closely resembling that of a 

 hardened slate or micaceous schist, and they are sometimes observed 

 to be traversed by strings or veins of the enclosing granite. The 

 frequency of the occurrence of black patches in this rock considerably 

 detracts from its value as a building-material, as may be seen in the 

 Museum building at Penzance, the front of which is constructed of 

 Lamorna granite containing numerous dark spots. In this quarry 

 the dark patches sometimes include imperfect crystals of quartz of 

 considerable size, resembling in all respects those of the surrounding 

 granite. 



When examined under the microscope the mica of this granite 

 appears to be chiefly of a dark brown colour, and is observed to be 

 marked by circular, or nearly circular, spots of a much darker shade ; 

 in the centre of many of these markings a crystal of apatite occurs 

 as a nucleus. This mica is often penetrated by well-formed crystals 

 of unaltered magnetite, which is also found, although with less fre- 

 quency, in the crystals of felspar and quartz. The presence of 

 white mica is somewhat exceptional ; but this granite contains light- 

 brown tourmaline as well as a few small crystals of apatite. 



The quartz of this rock encloses the usual liquid-cavities, which, 

 however, are not generally numerous ; the felspar is in part ortho- 

 clase, but a notable proportion of a plagioclastic species is also 



