JULY 30, 1897. ] 
southeast corner the Yellowstone National Park 
-barely falls within the area. The folio derives 
its name from the valley in which the Jefferson, 
Gallatin and Madison rivers unite to form the 
Missouri. The ‘Three Forks’ Valley is im- 
portant from an historic standpoint, as being 
the point which Lewis and Clark reached in 
July, 1805, when they named the three conflu- 
ent branches of the Missouri. 
The text begins with a general description of 
the geography and topography of the region, 
and then takes up the general geology. The 
oldest rocks in the region are the crystalline 
schists and gneisses, designated as of Archean 
age, which in pre-Cambrian time formed a land 
mass comprising nearly all the area included 
in the map. While the Algonkian beds were 
being deposited to the extent of from 6,000 to 
12,000 feet, there was a gradual subsidence of 
the whole region, and shallow seas for the most 
part prevailed. During the Paleozoic age there 
were many minor oscillations of the surface, 
which were more frequent during Cambrian 
time than during the deposition of the Devonian 
and Carboniferous limestones. Toward the 
close of the Cretaceous period a general eleva- 
tion began, which was accelerated after the depo- 
sition of the Laramie formation. The forma- 
tion of the mountain ranges, together with the 
subsequent erosion, resulted in many valleys, 
which eventually were occupied by fresh-water 
lakes. These lakes attained their greatest ex- 
tent in the Neocene period, lasting in all prob- 
ability until the Pleistocene period was well 
advanced, and during their earlier stages im- 
mense bodies of wind-carried volcanic dust 
were deposited in their waters, and are now 
seen as beds of pure white dust. At the same 
time the dust fell upon the surrounding country, 
from which it was afterward washed into the 
lakes, forming an upper series of yellowish and 
rusty-colored beds. These dust showers de- 
stroyed both animal and vegetable life, and the 
remains carried into the lakes were buried in 
their deposits, where they are now found as 
fossil bones and opalized and silicified wood. 
Under the ‘ Description of Rock Formations , 
are outlined all the formations from the Archean 
gneisses up through the Algonkian, Cambrian, 
Deyonian, Carboniferous, Juratrias, Cretaceous, 
SCIENCE. 
AT 
Eocene, Neocene and Pleistocene. The rocks 
of more than half of the area are of sedimentary 
origin, while the crystalline rocks occupy ap- 
proximately 1,000 square miles, the remaining 
third of the area being covered with igneous 
material. Prominent among the latter are the 
andesitic breccias which form the main part of 
the Gallatin Range, the great porphyritic lac- 
colite occupying the center of the Madison 
Range, and the basaltic plateau which lies 
west of the Madison Valley. 
Under the heading ‘Structural Geology,’ 
after a general consideration, the vertical and 
horizontal movements are discussed, and the 
development of the lake basins is described. 
The arrangement of the rock masses is com- 
plex, the structure being complicated by lacco- 
lites, dikes and surface flows of igneous ma- 
terial. Unconformities exist, showing that 
areas previously raised to land surfaces and 
worn down have subsided, have been crossed 
by an advancing shore, and later have passed 
beneath the sea. 
The lake basins are now the floors of exten- 
sive valleys separating the detached mountain 
ranges, which rise about 6,000 feet above their 
bases. As the lake deposits are at least 2,000 
feet in thickness, the difference of elevation 
between the bottoms of the lake basins and the 
summits of the peaks must be at least 8,000 
feet. The region was a mountainous one before 
the development of the lakes; but in the evolu- 
tion of the existing relief, movements and ero- 
sion have both operated to accent the topo- 
graphic differences. 
The principal economic resources of this re- 
gion are gold, silver, iron ore, copper, lime- 
stone and coal. The occurrence of coal in De- 
vonian rocks on the north side of Jefferson 
Cafion is of geologic interest, although not 
of much economic importance. The fine 
pumiceous volcanic dust found in the old lake 
basins has been utilized to a very limited extent 
asa polishing material. Brick clays occur and 
are used to a small extent ina number of locali- 
ties, especially near Bozeman. In addition to 
the economic resources just referred to, the 
sheet of economic geology has indicated upon it 
the localities of building stone and mineral 
springs. 
