Composition, and Analogies of Rocks. 200 



respect to the general ones, as the}'- have been produced 1)y that 

 gradual waste of the solid rocks which now form our superficial 

 alluvia, it is natural to expect that they should be found chiefly, 

 and most extensively, at the great interval which separates the 

 primary and secondary strata ; and this expectation is realized by 

 the existence of that almost universal conglomerate, the first 

 portion of that red sandstone, which is itself the lowest and first of 

 the secondary series. 



If no revolution of so general a nature can elsewhere be 

 traced, yet partial ones of an analogous kind are found both' in 

 the primary and secondary series ; and thus, in both, there exist 

 conglomerates which, if not universal, are still, in the sense here 

 laid down, entitled to the name of general. !nn/ 



The mechanical origin of all these rocks is so obvious, that it 

 is unnecessary to dwell on it ; while it is also easy to discover 

 that the component parts have undergone greater or less degrees 

 of attrition, and in many cases of transportation. It is also well- 

 known, 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 together. •^■ 



Those which consist of many different fragments, or even of 

 fragments of two substances, may be considered as general con- 

 glomerates. They are, in a geological sense, only modifications ' 

 of the different recomposed rocks with which they are found 

 associated ; and thus, like these, they necessarily occupy exten- 

 sive spaces in nature. They may thus be distinguished from the 

 local conglomerates, by their geological position 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 undergone, and by the variety of 

 ingredients they contain. These remarks apply principally to 

 those conglomerates which belong to the red sandstones, of which 

 they often form very conspicuous portions, as is universally known. 

 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 anv other* 

 Vol. XIX. P 



