Varieties of Potyliynj. 69 



rucks, ill spite of often possessing an extremely fine-grained 

 groundnmss, are in almost every instance holocrystalline. In 

 only rare instances was there a truly amorphous base present in 

 any appreciable amount. For this reason they will be classified 

 for description (in accordance with the foot-note on page 65) as 

 porphyries, porphyrites, and diabases. 



Porphyry. 



Micropegvmtite. — The absence of granitic rocks is noticeable 

 in the series examined from Alaska. The nearest approach to 

 this t3^pe appears to be number 13, collected l^y Professor Reid. 

 Both this and two other specimens which are related to it in the 

 structure of their groundmass, are, however, more basic than we 

 should expect true porphyries to be. They none of them con- 

 tain any porphyritic quartz. Their phenocrysts are altogether 

 hornblende and feldspar (mostly striated) ; so that, in the ab- 

 senceof a comjDlete analysis, they might perhaps be better classi- 

 fied as acid porphyrites. 



Number 13 is a pale gray rock of medium grain, which under 

 the microscope is found to be consideraljl}^ altered. Its por- 

 phyritic hornblende is largely changed to epidote and chlorite. 

 Its groundmass is rather coarse in texture, and consists almost 

 entirely of the intimate intergrowth of quartz and feldspar 

 known as micropegmatite. Allied to this specimen is number 

 7'' of the suite collected in situ by Mr Cushing. This rock has 

 rather an andesitic habit, consisting of porphyritic feldspar crys- 

 tals or groups of crystals imbedded in a groundmass of smaller, 

 but well-formed idiomorphic feldspars, niostl}^ striated. These 

 are not in actual contact, but are themselves connected by a still 

 finer groundmass of quartz and feldspar, nvhich are united in a 

 very minute micropegmatitic growth. The idiomorphic feldspar 

 forms a very large proportion of this rock. It is considerably 

 altered. Tiie ferro-magnesian constituents, whatever they once 

 were (mica or hornblende), are comparative!}^ rare, and are now 

 ahiiost completely changed to chlorite. 



The third specimen, related both in composition and structure 

 to the foregoing, is number 9 of Professor Reid's collection. Its 

 phenocrysts are altogether striated feldspars. Its groundmass is 

 rather fine grained and hypidiomorphic. The micropegmatite 

 here manifests itself in an abundance of pseudospherulitic tufts 



