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masses of land, I think it quite likely that the movements of the 

 ice took rather the form of rollitig than of gliding. That is to say, 

 in the case of the low ground I would liken the movements of the 

 ice to the movements of dust when driven along a road by the 

 wind, rather than to those resulting from the use of the broom. 

 Both kinds of results may, of course, occur separately, or may 

 occur combined, any number of times in the same section. 



Thickness of Drifts. — In regard to the thickness of the glacial 

 deposits at any given locality, I consider that thickness to be always 

 directly proportionate to the quantity of detritus contained in the 

 ice when it melted over that particular spot, less by the quantity 

 carried away by the action of the streams resulting from the melting 

 of the ice, as well, of course, as by the quantity removed by various 

 agencies in postglacial times. There are exceptional cases where 

 an unusual thickness of drift has been proved ; but there is usually 

 no difficulty in accounting for the excess by local causes. The 

 drift is nearly always thickest in the low grounds in the immediate 

 vicinity ol mountain areas : this is a priori exactly what might have 

 been expected. Its thickness in the low ground bordering on the 

 Solway was proved in the boring at Abbey Town, recorded by Mr. 

 Holmes {Quart. Jour. Geol. Soc, May, 1 88 1,) to be considerably over 

 a hundred feet. At this part, if I am not mistaken, the thickness of 

 the melting ice must have been something between 2,500 and 3,000 

 feet. Setting the thickness proved in the boring at one hundred 

 and fifty feet, and the thickness of the ice at 2,500, which is prob- 

 ably well within the mark, we find that there was a proportion of 

 detritus to ice of three to fifty. In other words : the quantity of 

 mud, dirt, and stones dispersed throughout fifty feet of ice was, 

 when it was collected all together, only equivalent to about three 

 feet. Had we been able to examine the ice at the time, we should 

 probably not have regarded the quantity of material contained in 

 it very excessive, as compared with the quantities known to occur 

 in bergs even at the present day. And yet this instance is an 

 extreme case where the sediment is unusually thick, and is inten- 

 tionally selected as such, 



