576 CONGLOMERATE ROCKS. 



most various and remarkable in the limestone^. 

 Hence their composition is regulated by that of 

 the rock to which they are united ; but, in some 

 cases, where they lie between two rocks of dif- 

 ferent natures, they contain fragments of both. 

 Such fragments are generally angular, and, in 

 many cases, so little displaced, that the imagina- 

 tion easily restores them to their proper situa- 

 tions. The geological difference of these and 

 the preceding, are therefore chiefly this that 

 the latter remain in the places where their inte- 

 grant parts were formed, while the constituents 

 of the former have undergone a transportation 

 more or less considerable. The simple fracture 

 and reunion of strata, account for the formation 

 of the local conglomerates ; while the general 

 have originated in geological revolutions of a 

 highly important and extensive nature. They 

 are the consolidated alluvia of a previous state 

 of the globe. The mineral differences consist in 

 the comparatively small number of the sub- 

 stances that enter into the local alluvia. These 

 also are frequently united by a distinct cement, 

 either of fine materials and therefore partly me . 



