396 W. R. BROWNE AND A. B WALKOM. 



andesites, which are almost devoid of such minerals. In 

 some cases, however, a good deal of alteration has taken 

 place, and it is possible that a good deal of the ferro- 

 magnesian mineral has been replaced by secondary material. 

 The norms of the two analyses of quite fresh rocks (andesite 

 and pitchstone) calculated according to the American 

 classification, show respectively 16*78 and 11*80 per cent, 

 of pyroxene ; from this it seems probable that there is a 

 good deal of ferro-magnesian constituent present in the 

 base though it cannot readily be distinguished under the 

 microscope. 



A. Plutonic. 



Grano-diorite. 



Ooarse-grained phanerocrystalline rock, consisting of 

 quartz, felspar, hornblende and biotite as far as can be seen 

 in hand specimen. Quartz and felspar form more than 

 half the rock. Under the microscope the rock is holo- 

 crystalline. Its grainsize is even and coarse, and the 

 grains have an average diameter of about 2 mm. The fabric 

 is hypidiomorphic granular. 



The minerals present are:— Plagioclase, orthoclase, 

 quartz, biotite, hornblende, magnetite, apatite and sphene. 



Plagioclase is the most abundant mineral; it is subidio- 

 morphic and is twinned after the albite law. It is a good 

 deal decomposed. The orthoclase is not so abundant but 

 is present in fair quantity. Both the felspars are crowded 

 with inclusions of tiny fragments of biotite, hornblende and 

 magnetite. Quartz is present but not abundantly. The 

 biotite is slightly decomposing to chlorite in places, and 

 has a rather fibrous appearance. The hornblende is the 

 green variety and is decomposing to chlorite. Apatite 

 and magnetite are fairly abundant and sphene is sparingly 

 present. The order of consolidation is : — 



