14 SUCCESSION OF STRATIFIED MASSES. [Ch. II. 



occasionally graduate into the secondary ; accordingly, an at- 

 tempt was made, when the classification of rocks was chiefly 

 derived from mineral structure, to institute an order called 

 transition, the characters of which were intermediate between 

 those of the primary and secondary formations. Some of the 

 shales, for example, associated with these strata, often passed 

 insensibly into clay slates, undistinguishable from those of the 

 granitic series ; and it was often difficult to determine whether 

 some of the compound rocks of this transition series, called 

 greywacke, were of mechanical or chemical origin. The 

 imbedded organic remains were rare, and sometimes nearly 

 obliterated ; but by their aid the groups first called transition 

 were at length identified with rocks, in other countries,, which 

 had undergone much less alteration, and wherein shells and 

 zoophytes were abundant. 



The term transition, however, was still retained, although 

 no longer applicable in its original signification. It was now 

 made to depend on the identity of certain species of organized 

 fossils ; yet reliance on mineral peculiarities was not fairly 

 abandoned, as constituting part of the characters of the group. 

 This circumstance became a fertile source of ambiguity and 

 confusion; for although the species of the transition strata 

 denoted a certain epoch, the intermediate state of mineral 

 character gave no such indications, and ought never to have 

 been made the basis of a chronological division of rocks. 



Order of succession of stratified masses. All the subaqueous 

 strata which we before alluded to as overlying the primary, 

 were at first called secondary ; and when they had been found 

 divisible into different groups, characterised by certain organic 

 remains and mineral peculiarities, the relative position of these 

 groups became a matter of high interest. It was soon found 

 that the order of succession was never inverted, although the 

 different formations were not coextensively distributed ; so that, 

 if there be four different formations, as a, b, c, d, in the annexed 

 diagram (No. 1), which, in certain localities, may be seen 

 in vertical superposition, the uppermost or newest of them, 



