II. 



Forms of Mountains. 



THE origin of mountains has been so fully discussed by different 

 writers that a paper treating of mountain-forms might be con- 

 sidered unnecessary ; but in these days, when an intelligent apprecia- 

 tion of the leading features of our earth's crust is an essential part of 

 a liberal education, a few notes upon the outlines of mountains and 

 of our British hills may be welcome to readers of Natural Science. 



Taking from the shelves of a library Jukes's " Manual of 

 Geology," in that battered, dog's-eared, and dirty condition which is 

 the highest tribute to the value of a book, we find hills classified as 

 (i.) hills of accumulation, (ii.) hills of upheaval, and (iii.) hills of circum- 

 denudation, though it is explained that the second and third classes 

 cannot exist alone, as upheaval is necessary for the elevation of the 

 block from which the hills are subsequently carved, and all hills 

 have undergone some denudation. Consequently we may confine our- 

 selves to a consideration of (i.) and (iii.), and of these (i.) may be 

 dismissed in a few words. 



The principal hills of accumulation are volcanoes, and an 

 interesting account of their forms will be found in a paper by 

 Professor J. Milne. 1 He brings forward evidence to show that many 

 regular volcanoes have " a form mainly due to the simple piling up of 

 material," and that they are not to be regarded "as cones which 

 have been subsequently modified by a number of secondary causes.' 

 Consequently the surface presents a curvature similar to one 

 exhibited by a pile of loose dirt " which would be produced by a 

 simple logarithmic curve revolving about an axis." Of course, such 

 a curve might be subsequently modified by denudation, as Professor 

 Milne remarks, and since all our ancient British volcanoes have had 

 their ancient surfaces entirely destroyed by the action of denudation, 

 we need not dwell further upon this class of mountain, but proceed at 

 once to consider the peculiar features of mountains of circum- 

 denudation. 



Taking a broad view of the action of denuding agents, we may 

 divide them into two classes, those which act in the dry way and in the 

 wet way respectively. The first class is responsible for the main out- 

 lines of mountains of frost-bound and desert regions ; the second, for 



1 Geological Magazine, 187S, p. 337. 



