This great desert is 100 miles wide, and on the 

 west is again abruptly terminated by the lofty 

 snow-clad range of the Sierra Nevadas, 500 miles 

 distant from the Rockies, and rivalling the lat- 

 ter in elevation, as they exceed tliem in the 

 grandeur and diversity of their scenery. Their 

 greatest altitude is upon the eastern side, where 

 they form an abrupt wall facing the Great Basin, 

 and rise to a height of 14,000 feet. But to the 

 west the slope is more gradual, descending slow- 

 ly to the low land of California, which, however, 

 is again separated from the Pacific by the chain 

 of the Coast and Cascade ranges. 



From the description of the physical features 

 of the Cordilleran system the lecturer passed on 

 to sketch its geological history , which is truly 

 wonderful. 



It was first pointed out that, alike in the 

 Kockles, the Wahsatch and the Sierra Nevada 

 ranges the core of the mountains is composed of 

 the most ancient or Archtean rocks, and that 

 these, prior to Cambrian times, formed a 

 sort of semi continent in the far west, the land 

 being mostly of a mountainous character, and 

 involving peaks and precipices miles in height, 

 which, in magnitude, are now only rivalled by the 

 topography of the moon. 



In the Palceozoic ages which succeeded the 

 Archaean, this lofty western land mass would 

 seem to have slowly subsided until about 30,000 

 feet of sedimentary beds, of marine origin, 

 had been deposited upou it, burying all but its 

 highest summits. The immensely long period of 

 time required for such deposition seems to have 

 been unattended with any greater physical dis- 

 turbances, but from its close ana onward the 

 evidences of such disturbances are frequent and 

 upon a scale of unequalled magnitude Thus, at 

 the close of the Carboniferous period, or end 

 of the Paloeozoic ages, we tind a movement oc- 

 curring by which western Nevada, with portions 

 of Oregon, previously dry, sink far below the sea 

 level, while, simultaneously, eastern >evada, 

 western Utah and Idaho rise above it. The 

 movements occurred along lines of fault, one of 

 which, now forming the eastern wall of the 

 Sierras, is 309 miles long, and marks a dislocation 

 by which the Nevada country has been relatively 

 dropped 3,000 to 10,000 feet. 



The next great movement was at the close of 

 the Jurassic era, and after 20,000 feet more of 

 sedimentary beds had been deposited over the 

 areas which were in a position to receive them, 

 ^gain we find a change in the disposition of land 

 and water. The Oregon-Nevada basin now dis- 

 appears by the rising of its bed, and simultaneous- 

 ly the Pacific coast subsided beneath the waves. 

 So in the early Cretaceous or chalk era there was 

 a renewal of disturbance, in this instance so vio- 

 lent that over an area embracing thousands of 

 square miles the rocks were so shaken and shat- 

 tered that the fragments produced do not on an 

 average exceed a hen's egg in size. The Coast 

 and Cascade ranges also date their beginnings 

 from this time. 



Still later, at the close of the Cretaceous, came 

 other movements resulting in the partial sub- 

 sidence of the Coast Range, reducing it to a 

 chain of islands, while east of the Wahsatch and 

 thence to the Rockies the land rose from 30,000 to 

 40,000 feet, thus completing the "Pacific slope,'' 

 at the same time that to the eastward of the 

 Rockies the region of the Great Plains became for 

 the first time dry, and, by the disappearance of 

 the old Inter-American ocean, eastern and west- 

 ern America became united into one great con- 

 tinental land mass. 



From this time onward we find no further de- 

 pression of the west below the sea level. There 

 were, however, frequent movements and a con- 

 tinuation of the old system of dislocation, further 

 breaking up the surface, while in the hollows 

 thus formed were produced a series of great 

 inland lakes, bordered by extensive forests, whose 

 shades gave shelter to vast numbers of gigantic 

 mammals and other forms of life. But in time 

 this condtion also passed away. The lakes were 

 drained off as the mountains rose— aridity follow- 

 ed upon excessive humidity— the rivers dried up 

 or became shrunken to narrow defiles, like that 

 of the Colorado, and indescribable deserts began to 

 take the place of what before had been a region of 

 universal verdure. At the same time, along the 

 lines of fracture, great floods of lava welled up 

 from below, to spread in molten torrents over 

 vast areas, and helping still further to make un- 

 inhabitable the already sterile wastes. 



But, with this disappearance of all that would 

 tend to make the surface suitable for tillage and 

 for man's habitation, came also the storing of 

 the fissured rocks with mineral veins, |making the 

 region the most productive mineral region in the 

 world. For, from the same region, up to the year 

 1880, there had been removed, in gold, lead, silver 

 and quicksilver, a total product of not less than 

 2,000 millions of dollars. The lead and silver are 

 chiefly found along the line of the old Wahsatch 

 fault— a fault by which the western half of the 

 chain had been dropped some 40,000 feet,while the 

 gold belt of California lies along the western 

 flank of the Sierra Nevada, where a similar dis- 

 placement of many thousands of feet has been 

 shown to have taken place. Lastly, the great 

 quicksilver deposits, without which the gold 

 could hardly have been discovered, lie nearer to 

 the coast, and undoubtedly owe their origin to the 

 powerful movements and accompanying igneous 

 phenomena by which the latter had been affected. 

 The lecture closed with some f urtler compar- 

 isons in the character of the movements distin- 

 guishing the three great mountain systems of the 

 continent, and the belief was expressed that, as 

 regards the Rocky Mountain system, these 

 movements were still in progress, and that 

 volcanic outbursts, upon an extensive scale, 

 might at any moment renew in that region the 

 events which at no distant period marked its his- 

 tory. 



