Barr ell — Growth of Knowledge of Earth Structure. 165 



of Dana's comments upon it are contained in a few para- 

 graphs from this review (12, 142, 1876). 



' ' Captain Dutton presents in this paper the views brought out 

 in his article in volume viii of this Journal, with fuller illustra- 

 tions, and adds explanations of his theory of the origin of moun- 

 tains. The discussion should be read by all desiring to reach 

 right conclusions, it presenting many arguments from physical 

 considerations against the contraction-theory, or that of the 

 uplifting and folding of strata through lateral pressure. There 

 is much to be learned before any theory of mountain-making- 

 shall have a sufficient foundation in observed facts to demand 

 full confidence, and Captain Dutton merits the thanks of geolo- 

 gists for the aid he has given them toward reaching right con- 

 clusions. His discussions are not free from misunderstandings 

 of geological facts, and if they fail to be finally received it will 

 be for this reason. 



We here give in a brief form, and nearly in his own words, 

 the principal points in his theory of mountain-making as 

 explained in the later part of his memoir. 



Accepting the proposition that there is a plastic condition of 

 rock beneath the earth's crust and that metamorphism is a 

 ' hy drothermal process,' and believing that 'the penetration of 

 water to profound depths [in the earth's crust] is a well sus- 

 tained theory,' he says that great pressure and a temperature 

 approaching redness are essential conditions of metamorphism. 

 . . . 'The heaviest portion would sink into the lighter colloid 

 mass underneath, protruding it laterally beneath the lighter 

 portions where, by its lighter density, it tends to accumulate.' 

 He adds : ' The resulting movements would be determined, first, 

 by the amount of difference in the densities of the upper and 

 lower masses, and, second, by inequalities in the thickness of 

 the strata: the forces now become adequate to the building of 

 mountains and the plication of strata, and their modes of opera- 

 tion agree with the classes of facts already set forth as the 

 concomitants of those features.' 



The view^s are next applied to a sj^stem of plications. 'It has 

 been indicated that plications occur where strata have rapidly 

 accumulated in great volume and in elongated narrow belts ; 

 that the axes of plications are parallel to the axes of maximum 

 deposit; and that the movements immediately followed the 

 deposition' — the case of the Appalachians being an example in 

 which the accumulations averaged 40,000 feet. He observes : 

 'Wherever the load of sediments becomes heaviest, there they 

 sink deepest, protruding the colloid magma beneath them to the 

 adjoining areas, which are less heavily weighted, forming at 

 once both synclinals and anticlinals. ' 



With regard to this new theory, we might reasonably question 

 the existence of the colloid magma — a condition fundamental to 



