9498 THE AMERICAN NATURALIST. 
The granites are found in the Piedmont plateau region. They : : 
comprise even-grained and porphyritic varieties, and granite gneisses, E 
all of which are plainly intrusive. Each variety is clearly described E 
by the author, who also gives analyses of many types. Averages of i 
these analyses give the following figures : E 
SiO, Al,O3 FeO; MgO CaO NaO K,0 P 
Normal grani 69.67 16.63 1.28 AS 2.16 4.73 471 
Porphyritic xai 69.28 16.73 1.25 72 2.13 4:33 4:59 ; 
Granite-gneisses 73.76 14.52 1.03 a 1.14 4.16 4.63 Ro 
E 
The normal and porphyritic phases possess the same composition. a 
The gneisses, however, are more acid than these, while their percent- 2 
ages of Al,O3, CaO, and MgO decrease.* 
Gneisses of the Adirondacks. — The gneisses of a portion of the 
Adirondacks are briefly described by Cushing? in a report on the 
geology of Franklin County, New York. They comprise granite- 
gneisses with the composition of hornblende-granites, and pyroxene- 
gneisses. The latter consist of pyroxene (augite and hypersthene), 
plagioclase, orthoclase, some hornblende and quartz. Intermediate 
gneisses composed of hornblende and andesine, with augite and 
hypersthene as common accessory constituents, are also present in 
some localities. These are identical in their features with certain 
hornblende-gneisses derived from gabbros, but the author is inclined 
to separate them from the latter as of different age. Intrusive in - 
these gneisses and in the Grenville series of sediments which are so - 
well represented in the district are great dykes and masses of anor- - 
thosite, gabbro, granite, diabase, and syenite porphyries. The vari- 
ous types of most of these rocks have been described many times. 
The author adds new descriptions which serve to show that the types - 
are quite uniform over large areas. The syenite grades into granite, 
both rocks being regarded as differentiates of one magma. T | 
nites are composed essentially of orthoclase and albite or oligoclase 
in microperthitic intergrowths, augite, hypersthene or bronzite nd 
quartz. Hornblende is nearly always present to some extent. With 
the increase in this component the hypersthene diminishes. — The 
rock varies rapidly in composition and structure. All the intru 
except the diabases and porphyries have been subjected to | 
pressure and have yielded gneisses. 
1 Amer, Geologist, xxvii (1901), p. 199. 
? 18th Report State Geologist, Albany, N. Y., 1900. 
(No. E was mailed October 14) 
