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PEOCEBDIKGS OP THE GEOLOGICAL SOCIETY. [Jan. 25, 



is a very much greater proportion of broken shells. A third fea- 

 ture is the presence of shells of British species in the Upper Drifts in 

 many places in Clydesdale, while they have not as yet been found 

 in Arran. These shells are in a much less perfect state of preserva- 

 tion than those of the Arctic bed*. 



Fig. 2. — Section of the Clydesdale Drifts, 



a. Boulder-clay. 



b. Laminated clay. 



c. Shell -bed, containing Arctic 



species. 



d. Clay, sand, and gravel, with stones. 



e. Shell-bed, containing British species. 

 /. Upper Drifts, covered by surface soil. 



Theory of the Origin of the Beds. — The mineral structure and 

 fossil contents of these beds, and the contrasts which have been 

 stated, point to a formation under general conditions ahke over 

 wide areas, with variations in the mode of action in particular 

 places. Mr. Smith's happy generalization connected the Arctic 

 shells with the striated and transported stones, as both indicating 

 the prevalence of a cold, probably a subarctic climate ; and 

 there is now an accumulating weight of evidence and opinion in 

 favour of the original hypothesis of Agassiz, namely, a general 

 covering of ice rather than the agency of icebergs. My own opinion 

 has for several years inclined to the former Adew. A careful exami- 

 nation of the lake-district of England, and a comparison of the 

 markings there and in Scotland with those in Switzerland, have led 

 me to conclude that the regularity and persistency of the markings, 

 and other evidences of ice- action, could only have been produced by 

 an icy envelope in a constant state of advance. Now streams of 

 granite -blocks have emanated from the granite nucleus in the north 



* Mr. Smith, of Jordanhill, was the first, after many years of careful research, 

 to establish a division of the superficial beds of Clydesdale. He placed shell- 

 beds on two horizons, separated by beds witliout shells ; the lower contained 10 

 to 16 per cent, of Arctic species, the upper only British species. Active ob- 

 servers, following the course so ably marked out by Mr. Smith, have been en- 

 abled to make a more complete classification of the beds. The most zealous 

 and successful have been the Rev. A. McBride, of Ardmory, Bute, and the Rev. 

 H. W. Crosskey, of Glasgow. They have intercalated the laminated clay above 

 spoken of, as a constant member of the Clyde series, and have more carefully 

 distinguished the Upper Drifts, while they have added very largely to the fauna 

 of the period. The results of these researches and of many others relating to 

 the surface-evidences of ice-action in Scotland, with many valuable observations 

 of his own, have been brought together by Mr. Geikie in an able and compre- 

 hensive memoir, ' The Q-lacial Drift of Scotland ' : Glasgow, 1863. 



