12 Microscopic Structure of a Granitoid Quartz-jjorjjhyry. 



are a few crystals of a triclinic felspar (probably oligoclase), 

 exhibiting with polarized light the fine parallel lines characteristic 

 of the triclinic group of felspars. The orthoclase crystals often 

 show the apparent angle of 90°, and when small are well formed. 

 In one or two cases a faint internal structure, parallel to the sides 

 of the crystal, may be noticed, which I have attempted to represent 

 in Figs. 1 and 2; but in general the interior is clouded and 

 structureless. 



Silica. — The grains or " blebs " of silica sometimes show a slight 

 approach to the crystalline form, but are generally rounded and 

 structureless. 



With the 1-inch object-glass (mag. 55 diams.) the silica is seen 

 to contain an immense number of cells just coming into view with 

 that power. Sometimes they seem to lie in lines or rows in 

 different directions. So numerous are these cells, that in a section 

 with an area of one-hundredth part of a square inch there are 

 several hundred, so that in a square inch there are several thousand 

 within a plane bounded by the sides of the slice, which is very thin. 

 In a round grain, or sphere, of about ^cVth inch in diameter, there are 

 probably no fewer than ten thousand cells ! 



With the ^th object-glass and No. 2 eye-piece, magnifying 400 

 diarfts., nearly all the cells are seen to contain fluid bubbles ; which 

 are still better revealed with No. 4 eye-piece, magnifying 860 diams. 

 The bubble in each case seems to occupy about one-fourth of the 

 entire space of each cell. A constellation of these fluid cavities is 

 shown in Fig. 3, in which, along with the fluid cells, are two large 

 " stone cavities," containing apparently broken stony materials. 

 One of the fluid cells, tubular in shape, is remarkable for showing 

 three distinct little bubbles, which have probably been prevented 

 from uniting into one by intervening impediments or irregularities 

 in the walls of the tube itself. 



Mica. — The mica is as usual scarred, but the crystalline form 

 ill defined ; the prevalent colour is sap green, in which are occasional 

 black j)atches. Along with the mica are also a few patches of a 

 structureless green mineral, which much resembles the " chlorite " 

 of trap rocks, and which is undoubtedly a " secondary " mineral in 

 them. 



Iron. — Iron pyrites appears in a few instances in the form of 

 specks of a rich bronze to ruby colour, translucent, but not trans- 

 parent ; along with these are rare instances of black grains, some- 

 times in groups, which I assume, with some hesitation, to be magne- 

 tite. Assuming the pyrites to be a secondary mineral, the quantity 

 of iron originally in this rock must have been exceedingly small. 



On the whole this rock is a fair example of a large class of 

 quartz-porphyries in which the constituents of granite are present, 

 but which differs from a true granite in having a felsitic instead of 



