190 PROCEEDINGS OE THE GEOLOGICAL SOCIETY. [Mar. 5, 



round ; though it may fill with sediment one that had previously been 

 formed. 



Thirdly, jaeither do most of the Swiss lakes lie in lines of dis- 

 location. Por many reasons I do not believe that any one of them 

 among the high Alps or on their flanks can be proved to lie in lines 

 of mere gaping fracture. Let us consider the nature of such fractures. 

 In any country where the strata are comparatively httle disturbed 

 and lie nearly horizontally, if it be faulted, there is no reason why 

 the fractures should be open. In the Oolites, for example, in the 

 South of England, where faults are numerous, and in the New Bed 

 Sandstone of the central counties, there is generally a simple dis- 

 placement of the strata up or down, on one side or the other ; or, if 

 the. disturbance go beyond this, it is that along the sloping line of 

 fracture the beds on the downthrow side are turned up, and those 

 on the opposite side bent down, by pressure and slipping combined. 

 In more disturbed districts, like the Welsh Coal-measures, the same 

 phenomena are observable : witness, for instance, the numerous sec- 

 tions from accurate observation, drawn on a true scale, by Sir Henry 

 De la Beche, Sir William Logan, and others. Experience both 

 above ground and in mines proves the same. Most lodes are in frac- 

 tures, and many lie in lines of fault. In metamorphic, excessively 

 contorted, and greatly fractured districts like those of Devon, Corn- 

 wall, and Wales, the cracks, whether bearing metals or not, vary 

 from mere threads to a few fathoms in width. They are always 

 filled with quartz or other foreign substances, frequently harder 

 than the surrounding matrix. I have often traced lodes on the 

 surface, in Wales, by the hard matter filling the crack standing 

 in relief above the surface of the softer enclosing rock. In lime- 

 stone rocks the cracks are usually partly filled with crystallized car- 

 bonate of lime. Lines of fracture are not, therefore, for purposes of 

 denudation, necessarily lines of weakness, unless it happen that on 

 opposite sides of the fault hard and soft rocks come together, when 

 of course the softer rocks will wear away more rapidly, and generally 

 originate a straight valley. 



Again, in an excessively contorted country, such as the Alps, it is, 

 I believe, impossible, in consequence of that contortion, that there 

 should be gaping fractures now exposed to view. Assuming for the 

 sake of argument the sudden violent contortion of the strata of any 

 great tract of country, we shall see that the contorted rocks now 

 exposed at the surface, even if broken, would be most unlikely to 

 gape. 



The expression " elevation of mountains " conveys to the minds of 

 many persons the idea that the elevation has been produced by some 

 force acting from below, along a line in the case of a chain, and on a 

 point of greater or less extent when the mountains lie in a cluster, 

 as a whole, more or less dome-shaped. Such forces would stretch the 

 strata ; and when they could no longer stand the tension, cracks would 

 ensue, and many lines of valley are assumed to he in such fractures. 

 But in Wales, the Highlands of Scotlp,nd, and more notably in the 

 Alps, the strata now visible have been compressed and crumpled, 



