VARIATIONS OF TEXTURE IN IGNEOUS ROCKS 59 1 



somewhat different, on account of the more abundant con- 

 spicuous feldspar phenocrysts, which are, however, very small, 

 so that the rock resembles a miniature of 121 N., reduced three 

 or four times. Under the microscope the groundmass is seen to 

 have been originally glassy, but now is devitrified, and has a 

 cryptocrystalline structure. The phenocrysts attain a diameter 

 of about I™"", and consist of feldspar and hornblende. The 

 feldspar, upon optical examination, proves to be oligoclase. 

 The hornblende is mostly altered to calcite and chlorite. 



Alaskite (114 N.). — This in the hand specimen is a typical, 

 rather fine-grained, dense rock. Under the microscope the 

 structure is hypidiomorphic granular. The grains average i™'" 

 in diameter, and consist of quartz, orthoclase, and microcline in 

 somewhat equal proportions. The only dark minerals present 

 are a few grains of chlorite and pyrite. 



Horiiblendite {\2\ N.). — This is a coarse-grained rock, typical 

 of its kind, and also consists of green hornblende, with blotches 

 of epidote. Under the microscope the structure is seen to be 

 allotriomorphic. No other minerals are present save those men- 

 tioned, and the epidote is secondary to the hornblende. In the 

 section studied the grains average about 3™"^ in diameter. 



Analysis of structure. — The series of rocks just described, 

 from 117 N. to 119 N., inclusive, shows a transition, which has 

 at one end a typical coarse granular rock and at the other a por- 

 phyritic rock with a glassy groundmass. In the intermediate 

 stages the phenocrysts increase in numbers and somewhat in 

 size, while the orroundmass shrinks in volume and becomes also 

 coarser. In 123 N., which is a typical granular rock only 

 slightly removed from 117 N., these phenocrysts have increased 

 until they nearly fill the rock, while the small portion of ground- 

 mass which remains has coarsened so that the grains are a 

 millimeter in diameter ; thus the rock at first sight appears a 

 typical allotriomorphic granular rock, from which it is moreover 

 not far removed. 



The very slight variations of structure between any two adja- 

 cent members of this transition series shows that the differences 



