FRAGMENTS OP OTHER ROCKS CONTAINED IN GRANITE. 7 



The grey granite extensively quarried in the neighbourhood of 

 Penryn is almost entirely free from patches differing materially, 

 either in texture or colour, from the surrounding rock. Such 

 bodies are, however, to some extent, represented by occasional 

 aggregations of a few large flakes of mica. One of these nests, of 

 about the size and thickness of a shilling, from Carnsew quarry was 

 found to consist of a mixture of greyish-white and black micas 

 without any admixture either of quartz or felspar. 



The granites in the neighbourhood of St. Austell, the majority of 

 which are to a greater or less extent schorlaceous, and which are 

 sometimes marked by reddish-brown spots resulting from the per- 

 oxidation of ferrous compounds, are generally free from nests 

 differing in texture or colour from the surrounding rock; and although 

 several quarries in this district were examined, no specimens of in- 

 clusions were obtained. 



At Gready, in the parish of Luxulyan, a large quarry has been 

 opened upon a coarse-grained grey granite containing a moderate 

 amount of black and silvery-white mica, with occasional crystals of 

 schorl. This granite sometimes contains spheroidal or ovoid bodies 

 very dark in colour, and closely resembling water-worn pebbles of 

 fine-grained "greenstone"; these are firmly imbedded in the rock 

 and exhibit distinct and sharply defined outlines. 



An inclusion from the Gready quarry, one half natural size, is 

 represented in fig. 1, PI. I. 



An examination of thin sections of the coarse-grained granite from 

 this locality does not afford much information which might not be 

 obtained by a careful study of hand specimens with the aid of a 

 lens. 



The black and silvery-white micas are, however, seen to be often 

 interlaminated, while the former frequently encloses a considerable 

 amount of magnetite with occasional minute crystals of apatite. 

 The dark mica in this rock is affected by the deeper brown spots 

 previously referred to as occurring in the micas of other Cornish 

 granites, and the quartz contains the usual liquid-cavities. A 

 notable proportion of the felspar is triclinic, and the rock contains 

 a few small garnets. 



Sections cut from rounded pebble-like patches found in this quarry, 

 when examined under the microscope, are found to be, with perhaps 

 the exception of scliorl, composed of the same minerals as the 

 enclosing granite ; but the crystals are much smaller and the pro- 

 portion of black mica is larger. A large portion of the felspar 

 has been rendered cloudy by an opaque product of alteration ; but 

 wherever the crystals have retained their original transparency a 

 preponderance of triclinic felspar becomes apparent. An alteration- 

 product of a greenish shade is also present. 



The following analysis of a specimen of granite from Gready 

 quarry has, for convenience of comparison, been placed side by side 

 with one of a close-grained dark-coloured ovoid inclusion from the 

 same locality. The granite selected for analysis was that immediately 

 surrounding the inclusion of which the composition is given. 



