MOUNTAIN-MAKING. 753 



Ages. — After the crust had become thickened, by the earth's internal 

 cooling, through the ages, and had been stiffened also by the plication 

 and solidification, and partly the crystallization, of the strata of the 

 supercrust, geosynclinals became less a possibility, and therefore of 

 diminished extent ; and consequently the chief movement caused by 

 the ever-continuing lateral pressure was an upward one. Hence it is 

 that the mountain -chains received their great height so largely in the 

 Tertiary ; and hence, also, the vastness of the areas over the earth's sur- 

 face that were affected by single movements, such as the high-latitude 

 movements of the Quaternary. There was, also, a downward bending 

 over those higher latitudes, in the Quaternary, and another in the warm 

 parts of the oceans — the coral-island subsidence. But these bear the 

 character of the times, in the extent of surface involved, and are wholly 

 unlike the mountain-making geosynclinals of earlier time. It is prob- 

 able that the Pacific coral-island subsidence, or geosynclinal, was the 

 counterpart of the geanticlinals over the continents of the later Ter- 

 tiary and Quaternary. 



13. Fractures and Outflows of Igneous Rocks become numerous, 

 after the Crust has become too much stiffened to bend easily. — Great 

 floods of doleryte and trachyte were poured out over the Rocky 

 Mountain slope, after the close of the Cretaceous period. The pre- 

 vious plications and solidifications of the strata involved in the making 

 of the various ranges of mountains — the Sierra Nevada and the Coast 

 ranges on the west, and the Wahsatch and Cretaceous mountains on 

 the east — had left the crust firm and unyielding ; and, being too stiff 

 to bend, it broke, and out leaped the fiery floods. It had broken 

 at times before ; but at this time the fractures became much more 

 numerous, and the floods of rock more extensive. Moreover, from 

 this era appears to date the opening of the great volcanoes of the 

 Shasta range. In fact, the greater part of the volcanic eruptions of 

 the world are probably of Tertiary and later origin. 



Fractures giving outlet to igneous eruptions have probably been, in 

 all cases, consequences either (1) of catastrophe in a geosynclinal, as 

 in the Triassico-Jurassic areas of the Atlantic Border, or (2) catas- 

 trophe in a geanticlinal, when the crust was too stiff for geosynclinal 

 bendings, as over the Pacific slope of the Rocky Mountains ; and the 

 latter became far the most common, in the later part of geological time. 



The principles in the earth's evolution above presented, have been 

 elucidated, for the reason stated on page 737, by reference mainly to 

 facts from North America. If true for that continent, the same must 

 be law for all continents. 1 



1 For a fuller discussion of the subject here briefly presented, see a memoir in the 

 American Journal of Science for June, July, August, and September, 1873, vols. v. 

 and vi. 



48 



