52 PROCEEDINGS OF THE PHILADELPHIA MEETING 



the American theory. As a topic of geologic philosophy, dealing with 

 such vast elements of force, time, and mass, it was naturally attractive 

 to Le Conte, and formed the subject of his important official addresses. 

 While he perhaps did not add any original element to the theory, he gave 

 such clear analysis and attractive presentation of the arguments for 

 genesis of mountains by compression of areas, weakened by thick sedi- 

 mentation, that he must stand with Dana as a chief expounder. 



We now accept as fact the horizontal crushing of thick strata in the 

 production of the great mountains, but the efficient cause of extensive 

 compression is still a subject of study. Under the Laplacian hypothesis 

 the primary cause is the cooling of the superheated interior ; but, accord- 

 ing to physical laws and mathematical calculations, the radial contraction 

 produced by any possible loss of heat since C'ambrian time can account 

 for only a veiy small part of the superficial shortening. The amount of 

 contraction of the circumference of the globe in 100,000,000 years, due 

 to the greatest cooling thought possible, is about 10 miles, which is less 

 than the amount of compression represented by any one of the great 

 mountain systems. The circumferential shortening on any great circle 

 of the globe since Cambrian time can not be less than 100 miles. 



Under the planetesimal hypothesis the shrinkage of the planet is due 

 to increase of density. 



"The heat of the earth is supposed to have been developed chiefly by re- 

 duction of volume and by radio-activity, and the heat thus developed is one 

 of the forces which check further decrease of volume. Loss of heat is, of 

 course, a cause of shrinkage, but its effect is thought to be less than that of 

 molecular and sub-molecular rearrangements of the material of the earth, 

 resulting in greater density. The loss indeed may not be greater than the new 

 heat generated in the shrinkage." ' 



It is estimated that to produce the circumferential shortening of 100 

 miles would require a radial shortening of 16 miles.^ This seems im- 

 possible from mere loss of heat during recorded geologic time, but possible 

 by condensation of a porous globe built up by infall of cold matter. As 

 the hydrosphere and atmosphere are chiefly matter expelled from the 

 earth's interior, they represent reduction of volume, and some reduction 

 during post-Cambrian time may be credited to that cause. 



Concerning oscillatory or diastrophic movements, Le Conte, after giv- 

 ing the proofs of such great down-and-up movements in certain areas, 

 like the Colorado plateau, wrote as follows : 



"It must be confessed that the cause of these oscillatory movements is the 

 most inexplicable problem in geology. Not the slightest glimmer of light has 



^ Chamberlin and Salisbury's lutroductory Geology, p. 225. 

 * The same, p. 224. 



