374 C. A. SUSSMILCH AND W. G. STONE. 



mineral or minerals, in all the slides examined, primary 

 ferro-magnesian minerals were not detected except in the 

 case of a few flakes of biotite, included in the quartz and 

 felspar phenocrysts. The primary minerals present are 

 quartz, felspar, magnetite, ilmenite, and apatite. The 

 secondary minerals are chlorite, kaolin, calcite, sericite, 

 epidote, and iron ores in part. 



Quartz occurs plentifully and is practically confined to 

 the phenocrystic stage. The crystals vary in size (see 

 megascopic description) till some approach fragments of 

 microscopic dimensions, although the quartz enters but 

 sparingly into the composition of the groundmass. Idio- 

 morphic crystals are somewhat rare, the phenocrysts being 

 generally of an angular or irregular form, where not other- 

 wise rounded through corrosion. It also exhibits much 

 fracturing, fragments of the same phenocryst often having 

 slightly different extinction, but very little strain-shadow- 

 ing, indicating subsequent differential movement of the 

 fragments. This taken in conjunction with its angular 

 character would appear to be the result of fracturing prior 

 to the solidification of the rock, and due probably to change 

 of molecular stress, consequent upon changes of tempera- 

 ture within the magma, subsequent to the crystallization 

 of the quartz. Examples of typical corrosion are numer- 

 ous, the groundmass commonly penetrating the crystals as 

 tubular pockets or shallower bays, and often occurring as 

 rounded inclusions. It is possible that such rounding of 

 phenocrysts, inclusions and deep embayments of the ground- 

 mass, were formed during the crystallization of the quartz 

 phenocrysts, and were due to an unequal supply of silica 

 molecules from the magma, instead of being, as generally 

 considered, the result of solution of the quartz by the 

 magma. 



Felspar is somewhat more plentiful than the quartz, and 

 also ranges down to microscopic dimensions. It is often- 



