ivans. Nx VaAc. See. b+ Dec. § 
crest of the Sierra Nevada to the eastern foothills of the Rocky Mts., 
and with a north and south extension of thousands of miles in British 
Columbia, the United States and Mexico, we have an extraordinary 
display of the products of volcanic action. This is the great silver belt 
of the world, and is also rich in mines of gold, copper, lead, etc. 
Throughout all the Paleuzoic and Mesozoic ages this country was an 
unbroken, though not entirely unwarped, sub-marine or sub-aerial 
plateau, where the most continuous and extensive series of sedimen- 
tary rocks was deposited of which we have any knowledge. At the 
close of the Jurassic age the western portion of this region was folded 
up, to form the great chain of the Sierra Nevada and Cascade Mts., and 
along this line of fracture numerous volcanic vents were established, 
Lassen’s Butte, Mt. Shasta, Mt. Hood, Mt. Baker, etc., which have 
continued in intermittent activity to the present day. In Tertiary 
times the plateau east of the Sierra Nevada was broken up by a series 
of north and south fractures, resulting in the formation of the remark- 
able system of meridional mountain ranges which constitute the chief 
topographical features of the district. These mountain ranges are 
composed of blocks of Paleozoic limestones and sandstones—now 
converted into marbles and quartzites—set up on edge or at a high 
angle,—- or of volcanic materials which have welled up through some of 
the fissures. Along the lines of fractures are great numbers of hot 
springs, the representatives of thousands more which existed in former 
days, and to which we owe the great system of fissure veins of this. 
country :—hot water charged with mineral matter gradually depositing 
this and filling the channels through which it flowed. 
The volcanic rocks which have been poured out in so many places 
exhibit a great variety of physical and chemical characters, but have 
been grouped by RICHTHOFEN and ZIRKEL into five species—propy- 
lite, rhyolite, trachyte, andesite and basalt. Capt. DUTTON, who has 
given great attention to the volcanic rocks of the West, has distin- 
guished a larger number of kinds and has adopted a different classifi- 
cation. Aside from these massive rocks there is another group which 
constitutes a marked feature both in the topography and geology, and 
these are those which have been made the subject of Mr. JULIEN’S 
paper. They are generally soft in composition, often highly colored,— 
white, red, blue, green, gray or yellow—more commonly white, red or 
gray. They are often quite local and usually occupy the lowlands, 
frequently underlying much of the level surface between the mountain 
ranges; and their best exposures are seen in the banks of streams. 
which have cut these lowlands, There they are shown to be often hori- 
zontally bedded and sometimes interstratified with lacustrine sediments 
and sheets of basalt. Typical exposures of these rocks may be seen 
