KNOWLEDGE OP EAETH STRUCTURE 169 



Orogenic Structures. 

 Views of Plutonists and Neptunists. 



Orogenic structures are, as the name implies, those 

 connected with the birth of mountains. Nearly synony- 

 mous terms are deformative or secondary structures. 

 On a small scale this division embraces the phenomena 

 exposed in the rock ledge or quarry face, or in the dips 

 and dislocations varying from one exposure to another. 

 These structures include faults, folds, and foliation. On 

 a larger scale are included the relations of the differ- 

 ent ranges of a mountain system to each other, relations 

 to previous geologic history, relations to the earth as a 

 whole, and to the forces which have generated the struc- 

 tures. 



In order to see the stage of development of this subject 

 in 1818 and its progress as reflected through the publica- 

 tions of a century, more particularly in the Journal, it 

 is desirable to turn again to those two treatises emanat- 

 ing from Edinburgh at the beginning of the nineteenth 

 century and representing two opposite schools of 

 thought, the Plutonists and Neptunists. 



Playfair, in 1802, devotes nineteen pages to the subject 

 of the inflection and elevation of strata. 6 He places 

 emphasis on the characteristic parallelism of the strike 

 of the folds throughout a region, as shown through the 

 intersection of the folds by a horizontal plane of erosion. 

 He contrasts this with the arches shown in a transverse 

 section and enlarges on our ability to study the deeply 

 buried strata through the denudation of the folded struc- 

 ture. He argues from these relations that the struc- 

 tures can not be explained by the vague appeal of the 

 Neptunists to forces of crystallization, to slopes of orig- 

 inal deposition, or to sinking in of the roofs of caverns. 

 The causes he argues were heat combined with pressure. 

 As to the directions in which the pressure acted he is not 

 altogether clear, but apparently regards the pressure as 

 acting in upward thrusts against the sedimentary planes, 

 the latter yielding as warped surfaces. His method of 

 presentation is that of inductive reasoning from facts, 

 but he stopped short of the conception of horizontal com- 

 pression through terrestrial contraction. 



