HYPOGEIC WORK. 381 



■were made, and in the courses and character of the transporting currents 

 and waves. 



Further, the making of the geosyncline must have been attended in each 

 case by a pushing aside of the rock material in the earth's mass existing 

 beneath it, and an upward bulging, or a geanticline, over the region adjoining 

 on one side or the other. 



The more prominent theories of mountain-making now current are (1) the 

 Gravitation Theory and (2) the Contraction Theory. 



1. The Gravitation Theory. 



The Q-ravitation Theory was brought forward in its simplest form by 

 James Hall in 1859. According to it, the making of the preparatory geosyn- 

 cline, in the case of the Appalachians, was due to the gravitation of the 

 accumulating sediments, in accordance with the principle explained by Her- 

 schel, whose views he cites ; and the making of the flexures over the region 

 was due to the same cause ; that is, to the subsidence and not to heating 

 from below. In the same paper, the general conclusion already referred to 

 is drawn that a geosyncline of accumulation, like that of the Appalachians, 

 is a necessary preliminary in all cases of mountain-making. In 1847, Bab- 

 bage published the important principle (included in a paper read before the 

 Geological Society of London in 1834) that in deepening accimiulations of 

 sediments, heat rises from below into the pile as its depth increases, as ex- 

 plained on page 258, and that the subterranean heat causes changes of level 

 through the expansion and contraction of the rocks. 



T. Mellard Reade, after a study of the expansion of heated rocks of dif- 

 ferent kinds, adopting the views of Herschel and Babbage, attributes flexures, 

 and other effects attending mountain-making, not merely to the heat from 

 below indicated by the rising isogeothermals, but also to additional heat at 

 intervals from a succession of intrusions of igneous rocks consequent on the 

 conditions. He styles his theory "the origin of mountain ranges by sedi- 

 mentary loading and cumulative recurrent expansion," — recurrent because 

 of the successive igneous intrusions. He found for the rate of expansion of 

 average rock 2-75 lineal feet per mile for every rise of 100° F. The igneous 

 intrusions are said to occur generally along the axis or axes of the range in 

 process of construction. 



The principle that loading causes subsidence of the crust has been supplemented by C. 

 King (1876) with its apparent complement that unloading by denudation causes elevation, 

 — he holding at the same time that these effects take place in a solid globe. The elevation 

 of the Rocky Mountain area, during Tertiary time, is accordingly attributed by him to the 

 removal, through denudation, of a vast amount of material from the vicinity of the Colorado 

 canon, and from other parts of the mountains. 



"With regard to the view of King, and especially this example under it, Le Conte has 

 observed that the weight of the rock material elevated in the rise of the great mountain 

 area to a height of 4000 to 11,000 feet was vastly larger than the amount lost by denuda- 



