210 Dr. Mac Culloch on the Origin, Materials, 



Tho local conglomerates, on the other hand, may be distin- 

 guished by their much greater variety as a class, and by the much 

 more limited variety of their ingredients, sometimes consisting of 

 only one, occasionally of two, but rarely exceeding three. 



The general conglomerates are also commonly composed of ma- 

 terials agglutinated without an intervening cement ; whereas 

 most of the local rocks of this character consist of one or more 

 sorts of fragments united by a third cementing substance, or by 

 a cement composed of one of the imbedded ingredients. The local 

 conglomerates rarely occupy any considerable space, and are 

 often very limited ; while they are always attached to some simple 

 or compound rock, with which, in some part, they are intimately 

 united. 



As the general conglomerates form a separate and indepen- 

 dent set of strata, the local rarely form more than one bed ; and 

 are sometimes not even found in the form of a bed, constituting a 

 single lamina only, adhering to a parent rock, or an irregular mass, 

 in some other way connected with it. 



The general, frequently contain rounded masses, but the frag- 

 ments of the local are commonly angular, or little affected by 

 attrition. In many instances they are perfectly acute ; while 

 occasionally also, when of large size, they are found to be so little 

 moved from their places, or separated from each other, that the 

 imagination easily replaces the detached parts. 



These rocks have been sometimes distinguished by the names 

 of Breccia, while the others have been called pudding-stone ; but 

 as the term Breccia has also been very indiscriminately used, it 

 is not convenient to perpetuate its application where it is necessary 

 to be accurate. 



Circumstances occasionally visible in the secondary strata, and 

 more particularly in the calcareous, will explain the origin of these 

 iocal conglomerates. 



The beds of these are often found covered on the surface by 

 their own fragments, intermixed with minuter particles of the 

 same, or of clay. The imaginary consolidation of such a mass 

 would form a local conglomerate ; and thus it may be understood 



