Physiognomy of the District. 41 



structure of the mountain-masses and their position, and which 

 does not allow them to proceed farther in that direction, 

 or until a special relation occurs to cause a turning, by means 

 of which the previous transverse valley is converted into a 

 longitudinal valley , and now the structure of the rock or moun- 

 tain-masses proceeds in a parallel manner, instead of inter- 

 secting the valley as formerly. But every longitudinal valley 

 does not derive its origin from a transverse one. The longitudi- 

 nal valleys, in which these transverse valleys open, that do not 

 issue at the foot of the mountain-group do not always, through- 

 out their whole course, retain the character peculiar to them, 

 of running parallel to the structure of the mountain-masses, 

 but sometimes again become transverse valleys ; and hence we 

 cannot conclude from the course of a valley as it occurs in one 

 tract of country, as to its course in another, because, as has 

 been shewn, that course is susceptible of change in some cases. 

 It will appear in another part of this treatise,* how far an ac- 

 quaintance with the valleys can be made available in the 

 search for repositories of useful minerals. 



Of the elevations, which, if not produced, are yet separated 

 by the openings forming the valleys, the high ?nozintain -ridge is 

 the most important. It determines not only, as the water shed, 

 the chief stretch, but also the principal form uf the whole moun- 

 tain-group, and besides, for the most part, affords a very good 

 opportunity of forming an opinion of its structure. It must not 

 be expected that what has been previously termed the central 

 mass constitutes every where the high mountain-ridge, for this 

 would only be the case if it could be assumed that the present 

 external form of the mountains had been the original one. We 

 frequently find that the high mountain-rklge is composed of 

 slaty mountain-masses, while the granite makes its appearance 

 in extensive mountain-masses, in the ridges of one or some 

 times of both declivities of the mountain-group. Nor does the 

 high mountain- ridge always include the greatest heights. The 

 ridges of the lateral chains likewise afford an opportunity for 

 useful observations, and they not unfrequently present expo- 

 sures of those mountain-masses which are covered on the de- 



AJready published in iliis Journal, vol. x.wiii. \>. 894. 



