WARREN. — ALKALI-GRANITES AND PORPHYRIES. 287 



abundant minute quartz blebs. Locally the porphyritic texture 

 becomes more prominent. Flow structures are common and in 

 certain localities, such as on the top and northern slopes of Pine and 

 Wampatuck Hill, and on the top of Hemingway Hill, the flow structure 

 is very strongly and beautifully developed. Taxitic structures are 

 common, especially where the flow structures are most in evidence. 

 Spherulitic textures ma}' also be seen in many places, but the latter is 

 not a striking megascopic characteristic. Under the hammer the 

 rock is tough and breaks with a sub-concoidal fracture. It is finely 

 jointed, breaking up into small, angular, prismatic blocks. Along 

 many joints and fracture lines, quartz or calcite, or both have been 

 deposited. The large mass of aporhyolite lying northwest of Fox Hill 

 is more uniform and massive than the other occurrences and shows 

 little of the flow and taxitic structures. 



Microscopic characters. — Microscopically the rock shows no un- 

 usual features for this class of rocks and a brief description will suf- 

 fice. Thin-sections and the chemical composition show conclusively 

 that the aporhyolite l)elongs to the same series as the granite, etc. 

 Phenocrysts are few and irregular in distribution, feldspar predomi- 

 nates, quartz appearing only in the form of minute grains. The feld- 

 spar phenocrysts are as a rule, when not broken mechanically, fairly 

 sharp in outline, and of a square or rectangular form. The large ones 

 may measure as much as 2| mm. on a side but are usually considerably 

 smaller. They now consist entirely of a fine microperthite verging 

 toward cryptoperthite in places. They are often fractured and 

 broken apart. The quartz phenocrysts, when they occur, are small 

 and are apt to be rounded or irregular. They do not show marked 

 resorption. No dark silicates are developed as phenocrysts, and in 

 fact these are missing from the rock as a whole. The body of the 

 rock shows considerable variation in texture. No part of it is now 

 glassy, but much of it is so fine as to appear isotropic with low 

 magnifications and it is only with very high powers and strong light 

 that it is seen to be entirely crystalline. The greater part of the 

 aporhyolite is a fine, variously textured mixture of alkalic feldspar 

 and quartz. INIost of the feldspar appears to be microperthitic 

 although distinct albite laths can often be detected. Through this 

 mass is everywhere sprinkled, more or less irregularly, tiny grains of 

 hematite and magnetite, the latter commonly showing sharp octahe- 

 dral outlines. The other alteration products are, kaolin, serecite, 

 calcite, siderite and limonite. 



Spherulites made up of segments of finely fil^rous quartz and feldspar 



