THE MINING DISTRICTS OF CORNWALL. 337 



of flaky graphite. The amount of porphyritically imbedded quartz 

 is exceedingly small. 



Under the microscope this rock is seen to consist of an amorphous 

 base enclosing a few flakes of mica, and traversed by numerous 

 although somewhat indistinct crystalline planes ; it contains small 

 crystals of orthoclase porphyritically imbedded, and a few very small 

 grains of quartz. Some of the felspar crystals have become par- 

 tially replaced by the nebulous greyish green mineral before re- 

 ferred to. 



. IV. From Trelissick Creek, north of Carrick Roads. — This elvan 

 belongs to a less numerous class, of which the course approximates 

 more nearly to north and south than to east and west. Elvans 

 coursing in this direction have already been stated to be of less 

 frequent occurrence in the mining districts of Cornwall than in other 

 parts of the county. The composition of this rock, however, is very 

 exceptional, even for one of the class, as they do not generally 

 differ materially in the proportions of their constituents from those 

 found in east-and-west dykes. 



The width of this elvan, which penetrates a greyish slate, is about 

 30 feet ; and its colour varies from yellow or buff to a dark chocolate- 

 brown, in accordance with the less or greater degree to which the 

 iron present has become peroxidized. Its general appearance is 

 that of a rock composed of a large quantity of mica, with a little 

 felspar, enclosing occasional crystalline fragments of quartz. 



Under the microscope thin sections are seen to consist of a nearly 

 equal mixture of quartz, felspar, and brown mica, enclosed in a 

 felspathic base. The felspar is monoclinic ; and the quartz contains a 

 few small gas-cavities ; but no well-defined fluid-cavities containing 

 bubbles were observed. 



In addition to the four above-described elvans, of each of which 

 an analysis in duplicate was made, thin sections of thirteen others 

 were cut and examined ; but they in no case exhibited any 

 peculiarity not observed in either one or another of the fore- 

 going. 



The fluid-cavities of the quartz in elvans ure exceedingly nume- 

 rous, and are more frequently crystalline in form than those which 

 are found in granites ; as in the case of the latter rock, the fluid- 

 cavities contain vacuities of varying relative dimensions, while some 

 of them are without bubbles and apparently full. 



Much information with regard to stone- and glass-cavities, and to 

 the conditions under which they are severally formed, is to be de- 

 rived from a careful examination of the imperfect quartz crystals 

 enclosed in the more coarse-grained elvans. 



Fig. 1 (PI. XVI.) represents an imperfect crystal of quartz, magni- 

 fied 15 linear, enclosed in a specimen of elvan from Trerice quarry, 

 Crowan. It will be observed that no fewer than six separate intru- 

 sions of felspathic base have taken place into its substance ; in one 

 instance the enclosed base has become nearly detached in the form of 

 a pear-shaped bleb, while in another the almost totally separated 

 fragment has assumed a pseudocrystalline form. 



2a2 



