ON THE ORIGIN OF MOUNTAINS. 



volcanoes to the sea has been attributed to the percolation of sea- 

 water through fissures to the heated interior of the earth ; and if the 

 outlines of the continental areas are hues of weakness or fracture this is 

 important. The instability of the margins of many coasts is indicated 

 by raised beaches and submerged forests ; in our own island especially 

 on the western rocky and more mountainous shores.* 



In addition to the movements directly produced by volcanic 

 upheaval, it is also considered that the very material ejected from 

 volcanoes would cause cavities that might lead to subsidence. 



Then again, we know that nearly all the material worn from the 

 land is deposited in the sea near the land, especially near the mouths 

 of rivers. It is never carried far out to sea. And as we have mechani- 

 cally formed strata several thousands of feet in thickness, such 

 changes in the distribution of sediment may, it is thought, have some 

 minor effects on the processes of elevation and depression. 



The rude correspondence between the coast-lines of Africa and 

 Europe on the one hand, and that of the American continent on the 

 other, lias often been the subject of suggestive remark. Dana, in 

 commenting upon it, also upon the prevalent N.E. and N.W. 

 courses or trends of the land, and the dependence of the outlines 

 and feature-lines of the continents and oceanic lands upon these 

 courses, says : — " Such lines of uplift are lines of fracture, or 

 lines of weakest cohesion, and, therefore, like the courses of cleavage 

 in crystals, they show by their prevalence some traces of a cleavage 

 structure in the earth — in other words, a tendency to break into two 

 transverse directions rather than others.*' This quotation shows 

 that we have yet much to learn on the subject ere the matter 

 becomes simple. 



The sea has unquestionably formed most of the bays and head- 

 lands. But " the tapering southwards of South America has not been 

 caused by modem denudation. The shape is the result of changes 

 which occurred far back in time. For Patagonia, from the Andes to 

 the east coast, is a vast pebble-bed worn out of the Andes to the west, 

 and deposited by current and tide on their eastern flank. Upheaval 

 has occurred on both sides of the chain during the historic period, and 

 thus the present state of things is favouring extension rather than the 

 diminution of the area."f 



While there may have been a general sort of continuity in the 

 direction of the main lines of upheaval and depression, the changes 

 have manifestly been great in the physical geography of the earth in 

 past periods. Lyell has observed that " it is not too much to say that 

 every spot which is now dry land has been sea at some former period, 

 and every part of the space now covered by the deepest ocean has been 

 land."I 



* See Duncan, Op. cit. 



\ Duncan, Op. cit., p. 8. 



■ Principles of Geology," llch cl , Vol. I., p. 200. 



