262 STRUCTURE COMMON TO ALL ROCKS. 



we judge that this range was formed at the end of the Miocene. To 

 illustrate : in Fig. 230 (p. 257) it is evident that the strata a were first 



deposited in a horizontal 

 position, then tilted and 

 eroded, and b were de- 

 posited unconf ormably on 

 their eroded edges. The 

 age of this mountain, therefore, is younger than a and older than b. 

 Sometimes several movements of lifting are revealed. Thus in Fig. 

 231, the strata a were deposited horizontally, then tilted by mountain 

 formation and eroded, and b deposited horizontally and unconformably 

 on their edges ; then by a second movement b was lifted (and of course 

 a also at a higher angle than before), and c was deposited uncon- 

 formably on b. 



Now, by examination of mountains in all parts of the world, it is 

 found, as might have been expected, that all the highest mountains are 

 comparatively young, and that the oldest mountains are of moderate 

 altitude. The reason is obvious : young mountains are in the vigor of 

 youth, and perhaps still growing, while the oldest mountains are in the 

 last stages af decay. The oldest of our American mountains are the 

 low Laurentides ; they are almost gone ; they are pre-Cambrian. 

 Then follow the higher Appalachian ; they are pre-Triassic. Then 

 the still higher Sierra ; they are pre-Cretaceous. To mention some 

 foreign examples : the Alps has certainly risen 10,000 and the Hima- 

 laya 19,000 feet * since Eocene times ; for so high Eocene marine strata 

 have been found on their slopes. 



Other Phenomena associated with Mountains. — The essential phe- 

 nomena demonstrating the process of mountain formation are the 

 folded structure, the slaty cleavage, the thickness of the strata, and the 

 position along the borders of continents. But there are other phe- 

 nomena associated with mountains, which are well explained by the lat- 

 eral-pressure theory, and therefore confirm the theory. 



1. Fissures, Fissure- Eruptions, and Dikes. — The strong foldings 

 of mountain strata inevitably produce fractures. Often these fractures 

 extend down to the sub-mountain liquid, and this latter is squeezed 

 out by the enormous lateral pressure, through the fissures and out- 

 poured on the surface as great lava-floods — such as the great lava-flood 

 of the Northwest, and the Deccan lava-flood, already described on pages 

 210 and 211. The outpourings on the surface may be entirely carried 

 away by erosion, and the filled fissures through which they came may 

 be exposed as dikes ; or else the sub-mountain liquid may have been 

 forced into fissures which did not reach the surface, and these also 



* American Journal of Science, vol. xxxvii, p. 413, 1889. 



