GEOLOGY OF LITTLE ROCKY MOUNTAINS 423 
ascended by Professor Dana in 1875, who noted its character and 
height and briefly described* the porphyry, the specimens 
obtained by him being identical with those obtained last summer, 
as shown by comparison of the hand specimens. 
The rock is a syenite-porphyry of very pale-brown, nearly 
white color. Ina groundmass which can be seen by the eye to 
be very finely granular, there. are numerous phenocrysts of 
orthoclase from one-quarter to one-half inch long, bounded by 
the usual faces m (110), 6 (O10), ¢ (oOL), and often y (201); 
they are equidimensional in habit. No ferro-magnesian minerals 
are seen, but on weathered surfaces the rock has a rusty color 
due to the oxidation of a small amount of iron ore. 
In thin section the rock appears wholly made up of feldspars 
with a little interstitial quartz. An occasional opacite-like patch 
shows the existence of a former sparsely scattered iron-bearing 
mineral which from the shape of the patches and a consideration 
of the character of the rock seems most likely to have been 
egirite. The large feldspar phenocrysts are unstriated; they 
are quite fresh, sharply bounded, and present several points of 
interest mentioned beyond. From a consideration of the large 
amount of albite in the rock it is probable that they are soda- 
rich orthoclases. 
The groundmass of the rock in which the phenocrysts 
mentioned above are imbedded consists of equidimensional 
grains of albite, which give short rectangular cuts in the section. 
The average size of these grains is about one to one-half milli- 
meter in diameter. They show the albite twinning extensively 
developed and usually in very fine lamella; the lamelle often 
are interrupted and die out in wedge-shaped strips and then 
commence again; they appear remarkably like the albites which 
occur in the Litchfield eleolite-syenite from Maine, and which 
have been described by Bayley,’ only that they are not bent or 
broken. Very rarely the pericline twinning is seen and some- 
*Op. cit. This is the same rock collected and described"}by Dana, 1875 (Ludlow 
report), pp. 128, near top, 129 bottom, and 130 top. 
2 Bull. Geol. Soc. Am., Vol. III, pp. 231 to 252, 1892. 
