Present Aspects of Glacial Geology. 549 



approach the Clyde. 1 The distribution of Loch Doon granite 

 follows much the same direction; hence it was inferred that 

 the body of the drift travelled outwards from the land and 

 consequently could not be sea-borne. 2 This, together with the 

 very few recorded examples of shelly drift in the South of 

 Scotland, the highest being that of Airdrie, 3 and the previously 

 expressed opinions that the Scotch "Till" was the result of land-ice 

 stifled observation. 4 It was only by the acumen and industry of 

 Mr. John Smith, of Kilwinning, within the last year that these 

 enormous stratified beds at levels up to 1,061 feet have been proved 

 to contain in the Boulder-clay beds, shells often well preserved with 

 the epidermis intact: a discovery of the utmost importance, to be 

 welcomed as throwing much light upon some obscure points. 



We naturally ask ourselves why are the High-level shelly beds in 

 Ayrshire constituted of Boulder-clay, while those of England, Wales, 

 and Ireland are of sand and gravel ? After carefully examining all 

 the leading examples, the natural explanation that presents itself to 

 me is a very simple one, viz. the form of the ground where they occur. 



In England and Wales the High-level Shelly Drifts are on the 

 summits of hills or ranges where there is either little material for 

 the manufacture of clay, or the orographic position renders it 

 unsuitable for the deposition of fine muds. In Ireland the presence 

 of limestone gravel in large volumes tells the same story, but in 

 Ayrshire the High-level Shelly Boulder-clays near Muirkirk occur in 

 a wide embay ment or plateau, surrounded by higher land in which 

 the deposit of fine material derived largely from the Carboniferous 

 rocks might readily take place. There are together with local rocks 

 Highland schists found in the Drift, and these may most readily be 

 accounted for by floating-ice. The very considerable difficulty in 

 assenting to the necessary convolutions and diverse tracks of an ice- 

 sheet or sheets, varied in direction and volume from time to time to 



1 See Memoirs of the Geological Survey of Scotland, Explanation of 

 Sheet 7, p. 14. 



2 In the " Scenery of Scotland" (second edition), by Sir A. Geikie, a Drift map 

 is published in which the only shelly drift shown between the Clyde and the Irish 

 Sea is in the extreme south-west corner of Scotland. It is evident from this that 

 the important shelly drifts of Ayrshire were unknown at the time of publication. 



3 Mr. Dugald Bell, previous to the discoveries of Mr. Smith in Ayrshire, threw 

 such doubts upon the recorded instances of the occurrence of shells in the Drift of the 

 South of Scotland that the .stock example of Chapelhall, near Airdrie, which was 

 supposed to prove a submergence of about 500 feet, was omitted in the last edition 

 of Dr. James Geikie's well-known " Ice Age." A committee with Mr. Dugald 

 Bell as a member re-examined the locality, with a result entirely negative. This is 

 only another instance of the futility of negative evidence, for, as we see, not long 

 afterwards evidence of the most conclusive sort of the presence of sea-shells in 

 natural sections open to the world were found in Ayrshire up to double the height. 

 See " The Great Ice Age and Submergence," by Dugald Bell, Geol. Mag., Dec. IV, 

 Vol II, p. 322. 



4 In a paper read before the Geological Society of Glasgow, April, 1880, and 

 published iu its Transactions, vol. vi, pt. 2, pp. 264-276, I stated my opinion that 

 the Scotch Till and the Low-level Marine Boulder-clay of Lancashire are continuous, 

 and that some of the unfossiliferous Scotch Till had probably been formed under 

 glaciers where they debouched into the sea. This is rendered far more probable now 

 that we know that the mud of the Scotch Boulder-clay is marine. 



