Re-diezv of Recent Geological Literature. 
-o:> 
(Tivcnticth Annual Report U. S. Geol. Survey. Part iii pp. 257- 
581. With 42 plates and 43 figures.) 
The stratigraphy represents the same general conditions that are 
found in the rest of the eastern part of the Rocky mountain area of this 
state. One feature of especial importance is shown in the Little Beli 
area, namely, the overlap of the Cambrian beds from the Algonkian 
in the south to the Archean in the north. 
The present structure and altitude of the Little Belt mountains are 
due to an uplift and folding of the range as a whole, accompanied b_v 
a. contemporaneous intrusion of large igneous masses producing minor 
folding and faulting. The principal peaks are igneous and present a va- 
riety of types. Sheets, laccoliths, and bysmaliths are found, all throe 
forms grading into one another. Mr. Weed discusses at some length 
the origin of such intrusions, concluding that in this region the form 
is a function of viscosity of lava, of resistance of overlying beds, and 
of the ascensive force. 
* At least two periods of igneous activity preceded the uplift of the 
range. The uplift was accompanied by a third period of activity, 
which was immediately followed l)y the formation of fissure veins in 
which the ore deposits occur. Inferentially these ores are believed to be 
post-cretaceous. Secondary enrichment has played an unusually im- 
portant part in the development of some of the ores. 
Professor Pirsson's report embraces a detailed description of the 
petrography of the region, and a discussion of analysis and of esti- 
mated mineralogical composition. He classifies the igneous rocks into 
four groups : 
a. Granular nonporphyritic rocks, of plutonic origin, including 
syenites, monzonites, diorites and shonkinites. Yogo peak shows, as 
part of a single geologic mass, monzonite grading into syenites on the 
one hand and into shonkinite on the other. This connection is interest- 
ing in that it exhibits in the field the grouping which Brogger has sug- 
gested on theoretic grounds. Monzonite is not uncommon in the west, 
appearing in company with cither more feldspar or more basic types, 
and represents the portion of mean composition in a differentiated 
complex. 
b. Acid feldspatliic porpliyrics, the predominant rock of the lac- 
coliths and of a considerable portion of the dikes and sheets. These 
rocks present many transitions, the transitional masses being often of 
more importance locally than the commonly known types. 
c. Lamprophyres, occurring in small dikes and .sheets, and includ- 
ing mincttes, vogesites, and analcite basalts. Variolitic facies. shown 
by some of the minettes of this region, is described as of a spherulitic 
nature. In this interpretation the author agrees with that of Pohlmann 
for the variolitic facies of the kersantites of Thuringia, and differs 
from the general explanation of the structure offered by Roscnbuscli 
in his text-book. These minettes afiford no evidence of the action of 
absorbed aqueous vapors, as described by Cross and by Iddings for 
spherulites in acid glas.sy rocks. , 
