" GREENSTONES" OF WESTERN CORNWALL. 161 



fresh triclinic felspar, together with viridite, hornblende, apatite, 

 and magnetite. There are also indications of pseudomorphs after 

 augite ; but this mineral has been entirely replaced by various horn- 

 blendic and chloritic substances. This is probably the " compact 

 felspar " of Dr. Forbes. 



The upper portion of the Chapel Rock, immediately west of the 

 causeway leading to St. Michael's Mount, consists of a mixture of 

 felspar, hornblende, viridite, magnetite, and apatite. The felspar, 

 which is chiefly plagioclase, is little altered ; but pseudomorphs after 

 augite cannot be very distinctly traced, being represented by patches 

 of viridite and fibrous hornblende ; the magnetite is unaltered. This 

 rock contains a few crystalline granules of quartz, and very closely 

 resembles that at Battery Point. 



On the sea-shore of the mainland, nearly opposite St. Michael's 

 Mount, the felspar is less distinctly defined, but augite is more 

 readily detected. This rock contains but little apatite, and much 

 of the magnetite has been replaced by a greenish-grey mineral. 



Specimens broken from the nearest point of rock east of the last- 

 mentioned locality resemble those obtained nearer Marazion, but the 

 felspar has become still more decomposed; pseudomorphic forms 

 after augite may nevertheless be traced ; apatite occurs in small 

 quantities ; and magnetite is, to a certain extent, replaced. 



The rock immediately west of the elvan-course which enters the 

 sea a little to the east of this place consists of crystalline but much 

 altered felspar, with crystals and patches of augite in a compara- 

 tively fresh state ; in addition to which it contains green hornblende, 

 perhaps tremolitc, a little viridite, and partially replaced crystals of 

 magnetite ; no apatite could be detected by the microscope. t Sec- 

 tions prepared from a rock on the eastern side of the elvan-dyke 

 do not differ from the foregoing, excepting that the augite is 

 generally in a still less altered condition. 



Altogether twenty-three thin sections of crystalline rocks from 

 the shores of Mount's Bay were prepared and examined ; but as 

 none of them exhibits any peculiarity not observed in one or other 

 of those already noticed, it would be useless to enter into a length- 

 ened description of them. In order, however, to compare the che- 

 mical composition of rocks of this class, differing somewhat in cha- 

 racter and in various stages of alteration, analyses were made of 

 four carefully selected specimens, each of which may be regarded as 

 being to some extent typical. Although made in duplicate, the 

 mean results only are, for the sake of convenient comparison, given 

 in the following Table. The mineralogical characteristics of the 

 rock, as deduced from a microscopical examination of thin sections, 

 have also in each case been added ; for, although these have already 

 been generally described, it is desirable that the records of the two 

 series of investigations should, as far as practicable, be placed side 

 by side. ? 



m2 



