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XIX. — On the Boulder-Clay of Europe. By David Milne Home, Esq. 



(Plate XXXI.) 



(Read 19th April 1869.) 



" Boulder-clay" or " till," abundant in Scotland, and occurring also in Eng- 

 land, Ireland, and in some other parts of North-Western Europe, has long been, 

 and still is, a puzzle to geologists. 



Sir James Hall, about fifty years ago, in this Society, was the first to draw 

 attention to the deposit, by describing its composition, and endeavouring to ex- 

 plain its origin. He saw that it could not be included in either of the two great 

 classes into which rocks were then divided. It was a deposit sid generis, bearing 

 no resemblance to anything known, except a heap of rubbish, there being in the 

 arrangement of its ingredients no regard to specific gravity or size. 



Sir James Hall ascribed the deposit to diluvial agency, and attempted to 

 show how the transport of the boulders and pebbles in it, their rounded forms, 

 and the abrasion of rocks covered by it, might all be accounted for, by supposing 

 that great waves of the ocean had swept over the country from west to east, 

 scattering debris in all quarters.* 



This diluvial theory was generally accepted, and relied on as satisfactory, until 

 about the year 1840, when the "glacier" theory was started, suggested probably 

 by the discovery that many of the shells found, if not in the boulder-clay itself, 

 at all events in other pleistocene beds, alternating with it, bespoke an Arctic 

 climate. 



A strong impulse was given to this new theory by the publication of a 

 magnificent work, on the Swiss Glaciers, by Agassiz, and by an account of 

 a visit which was shortly afterwards made by that naturalist to Scotland, in 

 company with the late Dr Buckland. Both of these eminent men affirmed 

 that they had seen unmistakable signs of glaciers in almost every valley they 

 visited. Shortly afterwards, the late Principal Forbes, who had, by frequent 

 visits to the Swiss glaciers, made himself well acquainted with their action, went 

 to Skye, and discovered marks of ice on many of its rocks. He read a paper in 

 this Society, describing these marks ; and as the learned Principal was distin- 



* Sir James Hall's theory is explained by him in the following paragraph : — " I imagine 

 that a diluvial wave flowed at some remote period from a westerly or north-west direction, and 

 broke over our island ; that its magnitude was such, that a great body of its water crossing the 

 ridge of country which separates the two coasts, overwhelmed the district, discharging itself into the 

 German Ocean." — (Ed. R. S. Tr. vol. vii. p. 202. 



VOL. XXV. PART II. 8 H 



