Trans. N. Y. Ac. Sci. 54 Dec. 5: 



crest of the Sierra Nevada to the eastern foothills oi 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 Paleozoic 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 seea 



