James Geikie — On Changes of Climate. 547 



neath the ice, then it is hard to see what difficulty lies in the way of 

 accepting Agassiz's theory. For since Boulder-clay was formed 

 below the mer de glace, there must always have been a certain 

 amount of it underlying the ice, which, upon the melting of that 

 ice, would be exposed, the presence of the sea not being at all neces- 

 sary to its accumulation. As a matter of fact, Boulder-clay occurs 

 on the tops of hills which are quite isolated and separated by broad 

 deep valleys from other high grounds. Thus, on the tops of the 

 Ochils, which it will be remembered divide the broad basin of the 

 Forth from that of the Tay, I have met with Boulder-clay at a 

 height of 1500 feet above the sea. Upon the watershed of the 

 Eenfrewshire uplands, also, I have seen very respectable accumula- 

 tions of typical Till, filling up the hollows of the hills, and even 

 capping some of the broader-topped eminences themselves. The same 

 deposit was observed by my colleague, Mr. Croll, on the very crest 

 of the Pentland Hills at a height of 1617 feet.^ My brother also men- 

 tions the occurrence of numerous striated stones — the wreck of the 

 Boulder-clay — near the top of Tinto Hill, in Lanarkshire. Now all 

 the hills referred to (and many other localities might be enumerated) 

 are more or less isolated heights rising up from that broad tract of 

 low ground which separates the Highlands of the north from the 

 pastoral uplands of the south of Scotland. 



According to the hypothesis I at present refer to. Boulder-clay, 

 although produced upon the land, could only have been deposited in 

 the sea. Consequently, wherever Boulder-elay is found, there we 

 must infer the seaward margin of the ice-sheet to have been. But 

 if we in imagination depress below the sea the isolated heights just 

 mentioned, we may well ask whence the glaciers could have come 

 which are supposed to have pushed the Till forwards and upwards 

 upon those hills. A submergence sufficient to have carried down 

 below the waves the Ochil Hills, the Pentland Hills, Tinto Hill, and 

 the Eenfrewshire uplands, must have drowned all Scotland between 

 the northern Highlands and the southern uplands. It is, therefore, 

 physically impossible that the Boulder-clay lying on the crests of 

 the hilly districts of central Scotland could have been deposited there 

 from the snouts of glaciers entering the sea. A glance at the map 

 of Scotland will satisfy any one of this. Take, for example, that 

 section of the Ochils which overlooks the valley of the Earn. These 

 hills, as already remarked, separate the basin of the Tay from that 

 of the Forth, nevertheless the Till on their crests and slopes reaches 

 in places a considerable thickness, and contains many boulders of 

 mica-schist and other Highland rocks, intermingled with numerous 

 fragments of Old Eed Sandstone, and the various igneous rocks of 

 which the hills themselves are composed. There can be no manner 

 of doubt, therefore, that the ice which deposited this stony clay came 

 from the north — that it crossed the basin of the Tay, and thereafter 

 surmounted the Ochils, and made its way into the valley of the Leven. 

 If the Ochil Hills were depressed to the highest level attained by 



1 This was on the top of AUermuir Hill. The clay contained fragments of mica- 

 scMst and several other rocks, none of which belono'ed to the hill itself. 



