52 MR SMITH ON THE CHANGES 



to a great extent in the counties of Lanark, Renfrew, and 

 Dumbarton. It is equivalent to the carse clay of the 

 Forth and Tay, and must have been deposited at the bot- 

 tom of a tranquil sea, at such a depth as not to have been 

 disturbed by the agitations of the surface. The shells and 

 other marine remains with which it abounds are almost in- 

 variably found in the lower part of this bed, a circumstance 

 which can only be accounted for by supposing a sudden 

 depression, which has converted a half-tide deposit into a 

 deep-sea one. The testaceous animals have thus been en- 

 tombed alive in the beds subsequently formed, and their 

 remains are preserved with all the perfection of recent spe- 

 cimens. Associated with this clay, we frequently find ex- 

 tensive beds of pure gravel and sand also destitute of orga- 

 nic remains, although there can be no doubt of their marine 

 origin. Mr Lyell has observed the same thing in similar 

 beds in Sweden.* 



We must be cautious, therefore, in concluding that allu- 

 vial beds in which we do not find such remains, are fresh- 

 water ones ; and, of course, equally so in deciding on their 

 marine origin, till confirmed by the presence of their appro- 

 priate remains. 



These deposits are much more extensive, both as to the 

 amount of change of level and superficial extent, than has 

 been generally supposed. We have conclusive evidence 

 that the whole of the British islands have, at periods which, 

 geologically speaking, are by no means remote, been sub- 

 jected to changes both of depression and elevation. The 

 submarine forests which have been observed on so many 

 parts of our coasts, are proofs of the former kind of changes, 

 whilst those of elevation are evidenced by raised beaches, 



* Phil. Trans. 1836, pp. 11 and 15. 



