SECONDARY ROCKS. 33 



ral proofs, which are so convincing, that both the limestones 

 and sandstones have been formed under the same general 

 circumstances. Concerning the remains of fishes which oc- 

 cur in the carboniferous limestones nothing need be said, 

 as all that is known about them at present is contained in 

 the great work of Agassiz ; the result of whose examination 

 is, that they exhibit no features which in any way justify 

 the opinion, which was so hastily formed concerning them, 

 that they belong to genera inhabiting bodies of fresh water. 



In the Lothians, as in other countries, the examination of 

 the position of strata affords data for drawing conclusions 

 concerning the age of the mountain-chain which forms a 

 part of them. The period at which a mountain-chain has 

 been upraised, is fixed between the completion of the depo- 

 sition of its newest upraised strata, and the commencement of 

 the formation of those which are resting unconformably. That 

 this mode of determining the ages of mountains, is at once na- 

 tural and free from leading the observer into error, is evident, 

 by considering those laws which regulate the deposition of 

 sedimentary matters. Conclusive, however, though this mode 

 of ascertaining the age of mountain- chains may at first sight 

 appear, still it is by no means safe in every case to consider 

 that their upraisure is contemporaneous with the production 

 of the unhorizontal position of certain strata, which form the 

 low country on either side of them. Under fitting circum- 

 stances, strata may be deposited at all angles under 40 degrees; 

 and when the cause which produced the deposition of the 

 strata at such angles becomes progressively, as we depart 

 from the axis of the chain, more fitted for their deposition at 

 smaller angles, the difficulty of fixing the age of the group 

 becomes greater, and recourse must be had to other phe- 

 nomena exhibited by the rocks of the chain. If such be 

 the arrangement, the fact of the angles nearer the central 



vol. vii, c 



