384 DYNAMICAL GEOLOGY. 



along a line farther west should be added for South America. The agent 

 for such results must be the earth in its entirety. 



2. Location of the lateral pressure. — The surface layer of the globe 

 in which the pressure acts has recently been shown to be thin. In the 

 cooling and contraction of the crust, the lower part of the cooled portion, 

 enveloping the uncooled nucleus that had not begun to lose its heat or 

 contract, could not contract without breaking, and, therefore, the cooling 

 would put it into a state of tension, which would result in the opening of 

 fractures. For if a layer undergoing contraction is united to a non-contract- 

 ing or less-contracting layer, the contraction would necessarily produce ten- 

 sion and fractures. Thus the cooling crust must be made up of an inner 

 portion in a state of tension and an outer in a state of lateral pressure, and 

 the two portions are separated by a level of no strain. The outer is the 

 effective part in orogeny. The lateral pressure within it is greatest at the 

 surface, and diminishes downward. The thickness of the effective layer 

 depends on the length of the time that has elapsed since the solidification of 

 the earth at surface — the time when the strain was initiated. It was esti- 

 mated by Mellard Eeade as only two miles (1886). It has been mathemati- 

 cally discussed first by C. Davison, and afterward by G. H. Darwin and M. P. 

 Eudski, who sustain the contraction theory of mountain-making. Davison 

 made the thickness (1887, '89) 2-17 miles, supposing the elapsed time to be 

 100,000,000 years; and Darwin (1887), two miles, for the same elapsed time, 

 adding that ''the depth is proportional to the time since consolidation." 

 Davison, in a later "calculation (1894) based on the supposition that the 

 coefficient of dilatation is not constant, as he before had assumed, but 

 increases with the temperature," arrives at the more favorable conclusion 

 that, after 100,000,000 years, "the depth of the surface of zero-strain 

 would be 7-79 miles." He says further, that "if the material of the 

 earth's interior be such that the conductivity and coefficient of dilatation 

 are greater in it than in the surface rocks, or if initially the temperature 

 increased with the depth, the above figure must be still further increased"; 

 and adds, in conclusion, "that, consequently, calculations as to the alleged 

 insufficiency of the contraction theory to produce mountain-ranges are at 

 present inadmissible." It is therefore safe to assume, in view of the 

 dependence of mountain plications on lateral pressure, that the thick- 

 ness was fully sufficient for the orographic results ; and even in late 

 Archaean time great enough to make Archsean mountains of 8000 to 10,000 

 feet, such as the Adirondack and Black Mountains must have been before 

 subjected to denudation. 



Darwin states at the close of his paper (which follows Davison's in the Philosophical 

 Transactions), after deducing that contraction vanishes at a depth of 2 or 3 miles : " Thus, 

 in 10,000,000 years, 228,000 square miles of rock will be crumpled on the top of subjacent 

 rocks. The numerical data with which we have to deal are all of them subject to wide 

 limits of uncertainty, but the result just found, although rather small in amount, is 

 such as to appear of the same order of magnitude as the crumpling observed geologically. 



