Scientific Lectubes. 93 



some explanation of the origin of the Iiills and valleys of this nionn- 

 tain region. 



Had tills disturhed zone heen elevated above the ocean level, 

 without change or fracture, we should have had a grand series of 

 parallel, or nearly parallel, arching ridges formed by the anticlinal 

 axes. But this folding of the beds has necessarily weakened them 

 along the summits of the arches ; and in the slow process of their 

 emergence from beneath the ocean, these lines have been the first to 

 come within the action of the waves : the erosion of the valleys, then 

 begun, was continued by the abrasion of the broken edges, till we 

 have the broad depressions marking the place of nearly all the anti- 

 clinal axes. In some instances, where the beds are strong and heavy, 

 the arch has been preserved ; and in others, one half of the arch has 

 been worn away, leaving the other half. As a consequence of these 

 extensive erosions of the anticlinals, the synclinal axes remain as the 

 most prominent parts of the range ; and it is only necessary to refer 

 to the geological sections before mentioned, to prove this remark. 



It nowhere appears that this folding or plication has contributed to 

 the altitude of the mountains; on the other hand, as I think it can 

 be shown, the more extreme this plication, the more it will conduce to 

 the general degradation of the mass, whenever subjected to denuding 

 agencies. The number and abruptness of the foldings will depend 

 upon the width of the zone depressed, and the depth of the depres- 

 sion, Avhich is itself dependent on the amount of accumulation. We 

 have, therefore, this element of depression to consider, when we 

 compare mountain elevations with the thickness of the original 

 deposition. In all cases where the strata are more or less disturbed, 

 a large proportion of the sediment constituting the mountain range 

 will remain below the sea-level, as a necessary consequence of the 

 great accumulation. 



If the facts presented warrant the conclusions I have drawn from 

 them, I think we are able to deduce some general principles in regard 

 to the production of mountain ranges. To explain the existence of 

 any such range, we are to look to the original accumulation of matter 

 along a certain line or zone, the direction of which will be the 

 direction of the elevation. The line of the existing mountain chain 

 will be the course of the original transporting current. The minor 

 axes or foldings must be essentially parallel to the great synclinal 

 axis and the line of accumulation. The present mountain barriers 

 are but the visible evidences of the sedimentary deposits upon an 



