548 THE JOURNAL OF GEOLOGY. 



has a similar form, except that the fault-cliff looks westward 

 instead of eastward. It is true that the extreme asymmetry of 

 these two mountains was given them long after their origin and 

 by a different process to be presently described. But even 

 before this last movement they were probably asymmetric, though 

 in a less degree. The Appalachian is perhaps here again a typi- 

 cal mountain. Its long slope is to the west and its crest close to 

 the eastern limit. The Alps, the Appenines, the Carpathians, 

 and the Caucasus, according to Suess, are foreign examples of the 

 same form. 



There are many other interesting points of structure that 

 might be mentioned, but they are less significant of mode of 

 origin and therefore omitted in this rapid sketch. 



ANOTHER TYPE OF MOUNTAINS. 



I have given the main characteristics of mountains of the 

 usual type, of which the Appalachian, the Coast Range, the Alps 

 and Pyrenees may be taken as good examples. But there is 

 another type, different in structure and in mode of origin, to 

 which attention, I believe, was first called by Gilbert. It is 

 doubtful if they are found anywhere except in the Basin and 

 Plateau regions, and therefore the type may be called the Basin 

 region type. The Basin and Plateau regions are broken by 

 north and south fissures into great crust-blocks which by gravi- 

 tative readjustment have been tilted, i. e., one side heaved up and 

 the other side dropped down, so as to form a series of north and 

 south ridges and valleys. Each ridge rises by a long slope on 

 one side to a crest and then drops by a steep fault-cliff on the 

 other. The ridges therefore are extremely asymmetric but the 

 asymmetry is produced in a different way from that of the usual 

 type. In a word, these mountains seem to be the result of a ser- 

 ies of enormous parallel faults. Such faults are common every- 

 where, but do not usually give rise to any inequalities which may 

 be dignified by the term mountain : or if so at one time, have 

 since been levelled by erosion. But those in the Basin region 

 are on so grand a scale and so recent in time, that they form 



