182 Washington — Igneous Mocks from Eastern Siberia. 



kindness. The results are given in V above, with the analysis 

 of a Norwegian grorudite in VI for comparison. 



The Miask rock resembles that of Iskagan Bay, especially in 

 the ratio of the alkalies, though these are here absolutely a lit- 

 tle higher, as is to be expected, in view of the somewhat lower 

 silica. Ferric oxide and lime are also present in somewhat 

 greater amount, as is also alumina, but the two do not differ in 

 any radical respect, except in the purely physical one of size 

 of grain. 



It is of interest to note that the relation of the Miask granite 

 to the Fron grorudite is just about the same as was that 

 between the Iskagan comendite and the Varingskollen groru- 

 dite, especially in the respective alkali ratios. 



Quartz-porphyry. — A rock which resembles the preceding 

 is represented by a slightly worn beach pebble. It is very 

 fine-grained and porphyritic, the groundmass composed of 

 alkali-feldspar and quartz, with phenocrysts of the latter min- 

 eral visible. The general color is a light yellowish brown. 



The microscope shows that it is composed very largely of a 

 fine-grained, noncrystalline groundmass of alkali-feldspar and 

 quartz in almost equal amounts, with very rare specks of mag- 

 netite. Phenocrysts of quartz and of a somewhat microperthitic 

 feldspar, often in Carlsbad twins, are scattered through this. 

 The groundmass is highly micropegmatitic, and closely resem- 

 bles on a smaller scale the granite-porphyry of the Castle Moun- 

 tains.* 



It was not thought worth while to make a chemical analysis 

 of this rock, but from the microscopical examination it is evi- 

 dent that it is considerably richer in silica than the preceding 

 one, and that probably the potash and soda are present in about 

 equal amounts (molecularly). It may, therefore, be held to 

 belong to the same series as the comendite. 



Rhyolite. — A magma of a distinctly less siliceous character is 

 represented by a specimen of a very dense, dark gray rock, 

 sprinkled with many small phenocrysts of white orthoclase and 

 rare ones of quartz. In general appearance it much resembles 

 the old rhyolites of Massachusetts and other eastern states. 



Under the microscope it is seen to be far from fresh, com- 

 posed very largely of alkalic feldspar in irregular grains and 

 small phenocrysts, a few of which show twinning lamellae, 

 with extinction angles which correspond to albite or an acid 

 oligoclase. A few irregular quartz grains are scattered through- 

 out, but the total amount of this mineral is very small. There 

 are also shreds and specks of a black substance representing an 

 original ferromagnesian mineral, but this can have been pres- 



* L. V. Pirsson, Bull. 139, U. S. G-. S., p. 86, 1896. 



