Boox VIL] GROWTH OF CONTINENTS. 919 
_ The gradual evolution of a continent during a long succession of 
geological periods has been admirably worked out for North America 
by Dana, Dawson, Dutton, Gilbert, Hayden, King, Newberry, Powell, 
and others. The general character of the structure is extreme sim- 
plicity, as compared with that of the Old World. In the Rocky 
Mountain region, for example, while the Paleozoic formations lie 
unconformably upon the Archean gneiss, there is, according to King, 
a regular conformable sequence from the lowest Paleozoic to the 
Jurassic rocks. During the enormous interval of time represented 
by these massive formations what is now the axis of the continent 
remained undisturbed save by a gentle and protracted subsidence. In 
the great depression thus produced all the Paleozoic anda great part 
of the Mesozoic rocks were accumulated. At the close of the 
Jurassic period the first great upheavals took place. Two lofty ranges 
of mountains,—the Sierra Nevada (now with summits more than 
14,000 feet high) and the Wahsatch,—400 miles apart, were pushed 
up from the great subsiding area. ‘These movements were followed 
by a prolonged subsidence, during which Cretaceous sediments 
accumulated over the Rocky Mountain region to a depth of 9000 
feet or more. ‘hen came another vast uplift, whereby the Cretaceous 
sediments were elevated into the crests of the mountains, and a 
parallel coast-range was formed fronting the Pacific. Intense meta- 
morphism of the Cretaceous rocks is stated to have taken place. ‘The 
Rocky Mountains, with the elevated table-land from which they rise, 
now permanently raised above the sea, were gradually elevated 
to their present height. Vast lakes existed among them, in which, 
as in the Tertiary basins of the Alps, enormous masses of sediment 
accumulated. The slopes of the land were clothed with an abundant 
vegetation, in which we may trace the ancestors of many of the 
living trees of North America. One of the most striking features 
in the later phases of this history was the outpouring of great floods 
of trachyte, basalt, and other lavas from many points and fissures 
over a vast space of the Rocky Mountains and the tracts lying to 
the west. In the Snake River region alone the basalts have a depth 
of 700 to 1000 feet, over an area 300 miles in breadth. 
These examples show that the elevation of mountains, like that of 
continents, has been occasional, and, so to speak, paroxysmal. Long 
intervals elapsed when a slow subsidence took place, but at last a 
point was reached when the descending crust, unable any longer to 
withstand the accumulated lateral pressure, was forced to find relief 
by rising into mountain. ridges. With this effort the elevatory 
movements ceased. They were followed either by a stationary 
period, or more usually by a renewal of the gradual depression, until 
eventually relief was again obtained by upheaval, sometimes along 
new lines, but often on those which had previously been used. The 
intricate crumpling and gigantic inversions of a great mountain 
- chain naturally suggest that the movements which caused these dis- 
__ turbances of the strata were sudden and violent. And this inference 
