288 PROCEEDINGS OF THE AMERICAN ACADEMY. 



are common. They may be scattered through the mass or they may 

 make up almost the entire field, again they are arranged in bands 

 The spherulites contain abundant grains of hematite or magnetite, 

 or both, arranged parallel to the radiation, and the same minerals are 

 often thickly clustered about the margins of the splierulite. In 

 plane light one can detect the presence of a splierulite by means of 

 these little rayed clusters of iron-oxide grains. Again the rock may 

 be largely made up of alternating bands of a fine irregular mixture of 

 the cjuartz and feldspar anfl of a material similar in structure to the 

 segments of the spherulites. The phenocrysts are often included in 

 these bands and hence follow flow lines through the rock. It seems 

 doubtful if the soda-iron silicates ever developed in the rock as origi- 

 nally solidified nor did they form on devitrification, but iron oxides 

 and silica separated instead. In some sections examined, a few 

 minute grains having the appearance of a hornblende, and a few tiny 

 specks of what is thought to be biotite have been noted, but these 

 are probably secondary. 



In some parts of the aporhyolite, micropoikilitic structures are 

 developed. These are especially strong in much of the rock from the 

 large mass lying northwest of Fox Hill, which, as has been noted, is of a 

 more massive character generally than the rest of the aporhyolite. 

 Over considerable areas of this occurrence the rock consists almost 

 entirely of a groundmass of micropoikilitic material enclosing fairly 

 numerous phenocrysts of microperthite. The micropoikilitic material 

 forms small roundish, elliptical or mutually moulded areas, which are 

 of about the same size as the feldspar crystals themselves — that is 

 from a few tenths to two or three millimeters, measured along their 

 greatest dimension. About the boundaries of these, magnetite or 

 hematite is often abundant and the same is sprinkled more or less 

 plentifully throughout the rock. The general effect with low powers 

 and crossed nicols is that of a rather finely granular rock. The micro- 

 poikilitic areas appear to consist of an intergrowth of microperthite 

 and quartz with a sufficient uniformity of orientation to give them 

 individuality. Some unorientated crystals of feldspar and quartz 

 are scattered through them. 



It is perhaps worthy of note that in some of the altered types of the 

 aporhyolite, siderite in tiny rhombohedra and irregular grains and 

 masses is sometimes quite abundant. The same mineral has been seen, 

 or strongly suspected, in the granite, identified in the porphyry, and 

 has been also found in some of the xenoliths in the granite. The 

 presence of this mineral as an alteration product is not common in 



