158 Barrett — Growth of Knowledge of Earth Structure. 



the view that the heat causing the change had ascended by the 

 very quiet method recognized in this theory. . . . 



But there are other facts indicating a limited sufficiency to 

 this means of metamorphism. These are afforded by the great 

 faults and sections of strata open to examination. In the Appa- 

 lachian region, both of Virginia and Pennsylvania, faults occur, 

 as described by the Professors Kogers, and by Mr. J. P. Lesley, 

 which afford us important data for conclusions. Mr. Lesley, an 

 excellent geologist and geological observer, who has explored 

 personally the regions referred to, states that at the great fault 

 of Juniata and Blair Cos., Pennsylvania, the rocks of the Tren- 

 ton period are brought up to a level with those of the Chemung, 

 making a dislocation of at least 16,000, and probably of 20,000, 

 feet. And yet the Trenton limestone and Hudson River shales 

 are not metamorphic. Some local cases of alteration occur there, 

 including patches of roofing slate; but the greater part of the 

 shales are no harder than the ordinary shales of the Pennsyl- 

 vania Coal formation. 



At a depth of 16,000 feet the temperature of the earth's crust, 

 allowing an increase of 1° F. for 60 feet of descent, would be 

 about 330° F.; or with 1° F. for 50 feet, about 380° F.— either 

 of which temperatures is far above the boiling point of water; 

 and with the thinner crust of Paleozoic time the temperature 

 at this depth should have been still higher. But, notwithstand- 

 ing this heat, and also the compression from so great an over- 

 lying mass, the limestones and shales are not crystalline. The 

 change of parts of the shale to roofing slate is no evidence in 

 favor of the efficiency of the alleged cause; for such a cause 

 should act uniformly over great areas." 



The next contribution to the theory of orogeny was a 

 series of papers published in 1873 by Dana, entitled "On 

 some results of the earth's contraction from cooling, 

 including a discussion on the origin of mountains and 

 the nature of the earth's interior." 11 This contribution, 

 viewed as a whole, ranks among the first half dozen 

 papers on the science of mountains. The following 

 quoted paragraphs give a view of the scope of this 

 article : 



"Kinds and Structure of Mountains." 



"While mountains and mountain chains all over the world, 

 and low lands, also, have undergone uplifts, in the course of 

 their long history, that are not explained on the idea that all 

 mountain elevating is simply what may come from plication 

 or crushing, the component parts of mountain chains, or those 



"This Journal, 5, 423-443, 474, 475; 6, 6-14, 104-115, 161-172, 304, 381, 



382, 1873. 



