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PREHISTORIC EUROPE. 



each other at all angles. The reasons for this are obvious ; the 

 stones which were being dragged forward under the ice would 

 naturally arrange themselves in the line of least resistance, and 

 this, in the case of the specimen here figured, would be length- 

 ways. Now and again, however, such a stone would be turned 

 over and get scratched to some extent in other directions. 

 Stones that had no particular shape would not of course travel 

 more easily in one position than another, and hence their 

 irregular striation. The boulders seem to have received their 

 finer polishing from being squeezed forward in the clay, which 

 acted upon them like emery. And doubtless the pavement 

 over which the stony clay was dragged was smoothed and 

 polished by the same agent. Not only hard rocks like granite, 

 but even soft black shales, which one may scratch with one's 

 finger-nail, have been rolled forward in their matrix of clay, 

 and in this position have acquired a finely - smoothed surface 

 upon which one may detect striae as delicate as the hairs of a 

 pencil- brush. 



Now, if boulder-clay has been formed in the manner I 

 have thus briefly described, we might expect that its origin 

 should be clearly shown by the mode of its distribution, by its 

 colour, and by the direction in which it has travelled. In 

 rugged mountain - glens, and on steep hill -slopes and hill- 

 tops — wherever, indeed, the ice moved with a quicker motion 

 than it could in broad straths and upon the open Lowlands, we 

 should certainly not meet with till in any quantity. It should also 

 be absent, or sparingly present, in all positions where, from the 

 configuration of the ground, there must have been enormous 

 force exerted by the ice. Thus, at the base of a steep hill front- 

 ing the direction from which the ice flowed, there should be little 

 or none — for the same reason that sediment gathers sparingly 

 in front of a boulder in the bed of a stream. But in the rear 

 of such a hill as I speak of, it is clear that, if our theory be true, 

 there ought to be a more or less considerable accumulation of 

 glacial debris, just as we expect to find gravel and sand heaped 

 up in the lee of boulders and submerged rocks in streams 



