1848.] SMITH ON SCRATCHED BOULDERS. 19 



valleys, moraines would be deposited, and fragments of rocks, de- 

 tached from the shores and resting upon, or entangled in the coast 

 ice, would be carried out to sea and dropt on its bottom at diiferent 

 depths ; but in this case the blocks would be found at lower levels 

 than the rocks from which they were detached. 



Mr. Darwin in a late paper has however shown, that boulders ire- 

 quently occur at a level considerably higher than their parent rocks, 

 and has accounted for it by supposing that they were floated to their 

 present position by ice during a movement of depression of the 

 land. 



Now we have, in the superficial beds in the basin of the Clyde, 

 evidences of such a movement which must have taken place in the 

 period when the climate was colder than at present, and which if not 

 paroxysmal was sufficiently rapid to have entombed alive the testa- 

 ceous inhabitants of the sea, and to have covered them up to a con- 

 siderable depth with beds of finely laminated clay, which could only 

 have been formed at the bottom of the sea. It is obvious that such 

 a movement must have had the effects ascribed to it by Mr. Darwin. 



In former communications I have shown, that the elevated marine 

 deposits in the superficial beds in this locality belong to two distinct 

 epochs, namely the newer pliocene or pleistocene, in which there is 

 a perceptible change in the fauna, and the post-pliocene, in which 

 the marine remains agree with those of our present seas. 



In the newer pliocene beds, the shells which are recent, but un- 

 known in the British seas, have all been found in the Arctic seas ; 

 here then we have evidence of a colder climate, and can thus account 

 for the presence of ice upon our shores. Now it is in these beds that 

 the proofs of depression occur. Beds of littoral and sublittoral shells, 

 such as the Mytilus edulis, are found to underlie beds of laminated 

 clay totally destitute of organic remains, which are sometimes thirty 

 feet in thickness, and seldom less than ten, except in cases where they 

 have been removed by the subsequent wasting action of the sea. 



In the shelly beds, the shells are generally speaking in situ ; the 

 bivalves with both valves adherent, still covered with epidermis, and 

 the borers in their vertical position. As there is no gradation from 

 beds in which the animals must have been alive when they were co- 

 vered up, to others totally destitute of organic remains, we cannot 

 ascribe their absence in the latter to the gradual process of decay, 

 but to an entire change of conditions, and that change must have 

 taken place with a certain degree of rapidity ; otherwise the shells 

 would have exhibited some evidences of the lapse of time which oc- 

 curred between the time when the animals were alive, and that in 

 which they were covered up. 



Under these circumstances, the ice upon the shores must have been 

 floated to a higher level, and with it the fragments of rock resting 

 upon it. 



I am satisfied therefore that Mr. Darwin has solved one of the 

 numerous difficulties which we encounter, when we attempt to ex- 

 plain the pheenomena of the erratic block beds. 



The same cause would also account for the position of the super- 



c2 



