106 SIR G. S. MACKENZIE on the Vertebra of a Whale. 



of its deposition must be very remote ; for the great mass of 

 gravel, which you saw the rivers of this county cutting through, 

 is a deposition subsequent to the clay which it covers ; and in 

 many places peat-mosses have been formed above the gravel. 

 You may remember my pointing out to you two distinct alluvial 

 deposites ; one consisting chiefly of clay, covering the sides of 

 the hills, and including enormous blocks of stone ; and the other, 

 consisting of gravel, which fills the valleys, and has been cut 

 through at different periods, as the succession of terraces tes- 

 tifies. I have seen nothing that certifies the comparative ages 

 of the marine clay and the oldest alluvion ; but the latter I con- 

 sider to be the oldest. It appears, therefore, that many centu- 

 ries must have passed since the sea retired from StrathpefFer, or 

 since the land was elevated. Whether the ocean be again ad- 

 vancing, I will not pretend to decide ; but I have not yet sub- 

 scribed to Mr STEVENSON'S theory. 



I am sure that the Society will be glad to preserve the bone, 

 as its having been discovered in circumstances nearly similar to 

 those under which the skeleton was found at Airthrie, and on 

 the same side of the island, render it extremely interesting. 



I am, 



My Dear Sir, 



Truly Yours, 



G. S. MACKENZIE. 

 COUL, \ 



21st January 1823. ) 



