546 James Qeikie — On Changes of Climate. 



gravel, sand, clay, silt, and mud ; containing in places Arctic shells, 

 and sometimes mammalian remains. 



2. Beds of gravel, sand, brick-clay, silt, and mud, with. Arctic and 

 Boreal shells abundant in maritime districts. Heaps of unstratified 

 or crudely stratified earth and clay, w^ith abundant angular un- 

 polished blocks and debris. The sand and gravel-beds frequently 

 assume the form of Karnes or Eskers. Erratic blocks. 



3. Moraines, and raised beaches. 



So much has already been written about the origin of the Scottish 

 Till or Boulder-clay, that it is perhaps almost superfluous to return 

 to this subject. There are still some geologists, however, who hesi- 

 tate to accept what appears to be the more generally received 

 opinion, namely, that the Till has been formed and accumulated 

 below a mer de glace. Quite recently Mr. Milne Home has sought to 

 resuscitate the iceberg hypothesis, and has collected a large amount of 

 evidence in support of this once favourite theory. He seems to me, 

 however, not to have sufficiently distinguished between the various 

 kinds of drift. It is admitted that during the deposition of the second 

 group, mentioned above, icebergs played a prominent part, but the large 

 erratic blocks, which are scattered up and down the country, together 

 with the whole of the Kame series, belong to a later date than the 

 accumulation of the Boulder-clay or Till. There is no fact about 

 which one can be more positive than this. But some of my geo- 

 logical friends who reject the iceberg theory yet find much difficulty 

 in admitting the feasibility of the views held by Professor Agassiz 

 and others. They cannot conceive how any accumulation of clay 

 and stones could take place underneath a mass of moving ice. The 

 ice-sheet, according to their notion, must always have scraped along 

 the surface of the bare rock, and the debris derived from this 

 scraping-process would, they think, be dragged along and pushed 

 out from below the ice-sheet upon the bottom of the sea. So that if 

 at any time during the period of greatest glaciation the ice could 

 suddenly have been peeled off the surface of the land, nothing but 

 bare rock would have been visible, from the highest points rubbed 

 by the ice down to the margin of the sea. According to this theory, 

 the Till could only have been deposited dm-ing a period of subsidence 

 — the Boulder-clay at the higher levels of the country having been the 

 last to be laid down. It cannot be denied that this hypothesis has a 

 plausible appearance, and that it does away with many difficulties 

 which the iceberg theory fails to remove. But it still leaves much 

 unexplained, and is moreover opposed to so many well-known facts, 

 that it cannot, I think, be maintained. 



It would lead me too far away from the matter more immediately 

 in hand were I to attempt to show in detail how this hypothesis 

 comes short of what is wanted. One or two points, however, may 

 be mentioned, which appear quite enough for this purpose. And, 

 first, it may be remarked that if the Scottish Boulder-clay be neither 

 iceberg droppings nor terminal moraine matter (at least in the same 

 sense as the debris at the foot of an Alpine glacier is), but has 

 actually been derived from the wear and tear of the rockhead under- 



