294 H. S. Summers: 



other into a quartz porpiiyry. The hand specimens show that 

 the rock is light grey in colour, and consists of phenoci-ysts of 

 felspar, with some quartz set in a fineir grained ground mass, a 

 comparison of the various types showing that as the groundmass 

 becomes finer in grain, the phenocrysts decrease in size. 



Under the microscope the rock is seen to consist of idio- 

 morphic iDlagioclase phenocrysts, with occasional irregular grains 

 of quartz, set in a hypidiomorphic to granular groundmass of 

 of quartz, plagioclase, orthoclase, with numerous flakes of mioa. 

 The felspar phenocrysts in general show good crj^stallographic 

 boundaries, but some are considerably corroded. A measure- 

 ment of the extinction angles of one of the phenocrysts, which 

 shoiws both carlsbad and albite twinning, proved that it was 

 andesine of the composition Ab^.An^, and therefore similar 

 in composition to .the plagioclases of the granite. A second 

 section which showed good zoning gave extinction angles, proving 

 that the central area was an acid andesine, and that the com- 

 position ranged from this to an acid oligoclase on the margin. 



The groundmass consists of quartz felspar and mica. The 

 quartz, which is clear and coloiu-less, occurs in rounded gi-ains, 

 in which are to be seen bubbles and included needles of apatite. 

 The felspar in the groundmass seems to be mainly plagioclase, 

 but some sections showing an entire absence of twinning are 

 probably orthoclase. By the use of Becke's method the re^ 

 fractive index of the felspars is seen to be generally about the 

 same, or lower than that of quartz, and higher than that of 

 the oanada^balsam, so that the felspars are either andesine or 

 oligoclase. Biotite is abundant, some being quite fresh, brown 

 in colour, and showing intense pleochroism, the remainder being 

 more or less converted to green chlorite. Included in the biotite 

 are small crystals of apatite and magnetite, suiTounded by pleo- 

 chroic halos. Magnetite has also separated out along the joint 

 planes in the ohloritized mica. 



(C). — llie Quai-tz-porphyry. 



The quartz porphyries are light grey to dark blue gi'ey in 

 colour, and show phenocrysts of quartz and felspar with oc- 

 casional flakes nf mica in a fine grained c:roundmas's. A section 



