DATE OF LAST GLACIAL EXTENSION 43 



other large game swarmed here.^then, and there was no 

 " channel " separating England from the Continent. We 

 know a long succession of events which have occurred 

 since then — the arrival of the Reindeer-men, their dis- 

 appearance, and the conversion of Europe into a pastoral 

 and agricultural land by the men of the polished stone 

 weapons, the arrival of the later bronze-using men, and 

 later still the introduction of iron. Most " prehistorians " 

 consider that much more than twenty thousand years has 

 elapsed since the last great glacial period covered North 

 and Central Europe with ice. Professor Penck, a very high 

 authority, estimates four hundred thousand years as having 

 passed since the first glacial extension of Pleistocene times. 

 But it is not possible in the present state of knowledge to 

 hold with conviction to any exact estimate, nor to be quite 

 sure that another glacial period is not already due ! 



The ice which forms by freezing on a lake or pond 

 differs a good deal in appearance and structure from 

 glacier ice. If a piece of lake or pond ice is melted in 

 warm air, the surface gradually liquefies, and the whole 

 remains clear. But if a piece of dense glacier ice from the 

 deeper part of a glacier (such as you may get from one of 

 the "ice-caves," often cut for show at the snout of a 

 glacier) is similarly melted, very fine cracks appear in it, 

 and gradually the lump breaks up into irregular crystalline 

 pieces. They are called " glacier grains," and are usually 

 about the size of a walnut, but may be smaller or bigger. 

 They are separate groups of ice crystals, and the glacier 

 ice is made up of these innumerable units tightly wedged 

 and fused together. Their origin is not properly under- 

 stood, but it appears (see p. 26) that the water which per- 

 colates the freshly formed nev6, and freezes so as to solidify 

 the mass, has more mineral matter in solution than have 

 the snow crystals themselves, and tnelts more easily (at a 

 lower temperature). Hence the sun's rays liquefy this 



