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no longer applicable in its original signification.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, 
