306 HISTORY OF GEOLOGY AND PAL/EONTOLOGY. 



range corresponded to the depth of a crust-hollow on the 

 neighbouring portion of the ocean-floor. 



This preliminary hypothesis is clearly open to question, 

 but a more important feature is Dana's assumption that the 

 centripetal movement of the crust, as it endeavours to shrink 

 along with the nucleus, is transmuted into tangential tension 

 comparable with the strains that would be set up in the case 

 of a falling arch. In Dana's opinion the horizontal pressure 

 components thus originated fold the crust into arched ridges 

 and trough-like hollows. Dana called the latter geo-synclinals, 

 the former ge-anticlinals ; and he applied the qualifying term 

 "monogenetic" to mountain-systems which owe their origin 

 to a single arch or ge-anticlinal such as the Uinta mountains 

 of Wyoming and Utah. On account of their frequent cracks 

 and fissures, monogenetic crests are rapidly lowered by the 

 action of subaerial denudation. 



The mountain-systems composed of several chains always 

 arise, according to Dana, within geo-synclinals where immense 

 masses of sediment have collected. As the older rock-horizons 

 become mantled by ever-increasing thicknesses of sediment 

 above, and the subsidence continues, the deeper strata are 

 weakened by heat and pressure and readily tear asunder. 

 The broken fragments yield to the horizontal pressures, are 

 crushed into a narrower space against the lines of tearing, 

 are folded and thereby uplifted. Dana called a mountain- 

 system elevated from a synclinal area of subsidence a 

 " synclinorium." The deeper geo-synciinals of past geological 

 epochs have been as a rule next the continents, and the new 

 mountains originated there slowly, the movements occupying 

 vast geological ages ; after their emergence they were incor- 

 porated with the main continental masses. 



Dana then discussed the conditions under which volcanic 

 rocks might take a dominant part in the building up of 

 mountain-chains. The earth's crust, he said, grew thicker 

 by the continued progress of cooling, and the rocks became 

 more and more resistant owing to the mechanical and 

 chemical metamorphoses which they experienced in the crust. 

 The process of mountain-making was consequently made more 

 and more difficult in the older areas of disturbance, but as 

 the tangential strain never relaxed, it might effect an upward 

 pressure of the crust, culminating in rupture, and allowing 

 the escape of volcanic rock at the surface. Hence the 



