228 ON THE ORIGIN, MATERIALS, COMPOSITION, 



undergone greater or less degrees of attrition ; and, in 

 many cases, of transportation. It has also been shown 

 in another place, that, with the exception of the Tuff 

 of the overlying family, they consist, in most instances, 

 of different ingredients; and, not unfrequently, of a 

 great number intermixed. 



Those which consist of many different fragments, 

 or even of fragments of two substances, may be con- 

 sidered as general conglomerates. They are, in a 

 geological sense, only modifications of the different 

 recomposed rocks with which they are found asso- 

 ciated ; and thus, like these, they necessarily occupy 

 extensive spaces in nature. They may thus be dis- 

 tinguished from the local conglomerates, by their 

 geological positions and connexions ; while they may 

 also, in a great measure, be recognised by their 

 mineral structure ; chiefly, indeed, by the attrition, 

 whether greater or less, which the parts have under- 

 gone, and by the variety of ingredients which they 

 contain. These remarks apply principally to those 

 conglomerates which are found among the secondary 

 strata, where different kinds or series meet, and, 

 above all, to the old, or lowest, red sandstone, of 

 which they often form very conspicuous portions. 

 Those which are connected with the Overlying rocks, 

 like the Tuffs of the same division, are distinguished 

 by such peculiarities of character as to admit of no 

 comparison with any others. 



The local conglomerates, on the other hand, may 

 be distinguished by their much greater variety as a 

 class, and by the much more limited variety of their 

 ingredients, sometimes consisting of only one, occa- 

 sionally of two, but rarely exceeding three. The 

 general conglomerates are also commonly composed 

 of materials agglutinated without an intervening 



