HYPOGEIC WORK. 383 



of the axis of disturbance, and none in the Laramide Range of British 

 America, and the same is true in a large part of mountain-making. In the 

 Wasatch the igneous effusions were a final effect, not an agent of change. 



Moreover, the pressure from any igneous intrusions, or their power of com- 

 pression, is feeble. Plastic rock is little better for pressure than any pasty 

 material ; when extruded it is hurried out of the way by the compression of 

 any other agent, or escapes, if it can, by gravity. When it cannot escape, it 

 bulges up the overlying beds and makes laccoliths (page 301), and this is 

 almost its limit of mechanical work. The heat also is wholly inadeqxiate 

 for plicating and faulting rocks in mountain-making style, whether the liquid 

 rock be granitic or of any other constitution; the laws as to heating and 

 cooling are the same for all kinds. 



2. The Contraction Theory. 



1. Tlie source of lateral pressure. — The source of the pressure accord- 

 ing to the contraction theory is the contraction of the earth's crust as a con- 

 sequence of cooling. The theory was suggested by Descartes in his Principia 

 PhilosopMce. in 1644, and by Newton in 1681, and was adopted in geology 

 by James Hall, of Edinburgh, in 1812, and advocated by De La Beche 

 in 1834. The contracting crust derives the lateral pressure from the cooling 

 and solidification that is going on underneath it — the crust being forced 

 to adapt itself to an interior which is becoming smaller by the earth's 

 gradual refrigeration. Mountain-making, according to the theory, is a con- 

 sequence largely of the earth's shrinkage. 



The author's contributions to the subject, including also that of the Origin of Con- 

 tinents and their Features, appeared first in the years 1846, 1847 and 1849, and were 

 continued in 1856 and 1873. The development of the structure of the Appalachians 

 through Virginia and Pennsylvania by the Professors Rogers afforded the first geological 

 demonstration in favor of the contraction theory ; and the results they published, although 

 leading the investigators at the time to a theory based on forced movements in the 

 earth's liquid interior, underneath a thin crust, afforded the author illustrations of the 

 views in his early papers. 



Since the earth has oceanic basins and continents of diverse dimensions 

 and features, this lateral pressure would work with direct reference to conti- 

 nental lines, and generally have its shoving and relatively resisting sides in 

 epochs of orographic work. If the pressure acted thus unequally from the 

 two opposite directions, it would make inequilateral mountain structures, or 

 those having a front-and-rear character, like the Appalachian Range. 



Moreover, the movements would have their limits determined by, or re- 

 lated to, the lengths of continents, or great continental regions, and, in this 

 respect, they accord with the actual characters of mountain chains. The 

 Laramide system, over 4000 miles in length, along the western continental 

 border of North America, is an example ; and perhaps another 4000 miles 



