2IO The American Geologist. October, looi. 
Ridge trachyte of the Rosita Hills, Colorado. Hornblende in 
this rock is chiefly green in ordinary light as seen in the thin 
section, though a few of the largest crystals are brown re- 
sembling the hornblende of the essexyte, while the green is 
like that of the nordmarkyte. Both are trichroic with the same 
scheme of absorption, viz. : c > b > n and extinction angles 
as high as 26°-27° have been observed in sections in the zone 
of the clinopinacoid. Augite when present is of the colorless 
variety described before. 
Biotite occurs in large individuals but is on the whole 
less abundant than hornblende. The groundmass consists 
chiefly of short rather stout prismatic sections of feldspar 
which have a parallel extinction and are packed together in 
a close and nearly parallel arrangement, with a little allotrio- 
morphic feldspar in the interstices. A little undetermined 
matter occurring interstitially amongst the feldspars was 
thought to be altered nepheline. It may, however, be only 
kaolinized orthoclase. A few striated sections appear which 
extinguish at low angles with the twinning lines and are 
probably oligoclase. They are unimportant in amount. The 
texture of the rock was found too fine to admit of a mechan- 
ical separation. 
Its mineralogical and structural characters at once suggest 
tlie affinity of this rock to pulaskyte, first described by Mr. 
J. F. Williams* from Arkansas, a view that is also corrobo- 
rated by the results of its chemical analysis. In the hand speci- 
mens it is generally somewhat lighter in color and finer in 
texture than the specimen of pulaskyte in the educational ser- 
ies of the United States Geological Survey at the petrograph- 
ical laboratory of McGill University, and judging from the 
published description it is very similar to the porphyritic 
syenyte from Saline county, Arkansas.* This, however, was 
considered by Williams as scarcely to be distinguished from 
pulaskyte, occupying the same position amongst the igneous 
rocks of Saline county that pulaskyte holds at Fourch moun- 
tain. 
A variety of this rock in which hornblende and augite are 
replaced by ?egerine-augitc approaches very closely to the sol- 
*"The Igneous Rocks of Arkansas," J. F. Williams. An. Kept. Geological 
Survey of Arkansas, Voi. II, 1S90. 
flbid, p. 140 and seq. 
