CHAP, yii 



THE GLACIAL EPOCH 



107 



other. The evidence for both these changes having oc- 

 carred is conclusive ; and as they must be taken account of 

 whenever we endeavour to explain the past migrations and 

 actual distribution of the animal world, a brief outline of 

 the more important facts and of the conclusions they lead 

 to must be here given. 



Proofs of the Recent Occurrence of a Glacial Epoch. — The 

 phenomena that prove the recent occurrence of glacial 

 epochs in the temperate regions are exceedingly varied, 

 and extend over very wide areas. It will be well therefore 

 to state, first, what those facts are as exhibited in our own 

 country, referring afterwards to similar phenomena in 

 other parts of the world. 



Perhaps the most striking of all the evidences of glacia- 

 tion are the grooved, scratched, or striated rocks. These 

 occur abundantly in Scotland, Cumberland, and North 

 Wales, and no rational explanation of them has ever been 

 given except that they were formed by glaciers. In many 

 valleys, as, for instance, that of Llanberris in North Wales, 

 hundreds of examples may be seen, consisting of deep 

 grooves several inches wide, smaller furrows, and striae of 

 extreme fineness wherever the rock is of sufficiently close 

 and hard texture to receive such marks. These grooves 

 or scratches are often many yards long, they are found in 

 the bed of the valley as well as high up on its sides, and 

 they are almost all without exception in one general direc- 

 tion — that of the valley itself, even though the particular 

 surface they are upon slopes in another direction. When 

 the native covering of turf is cleared away from the rock 

 the grooves and striae are often found in great perfection, and 

 there is reason to believe that such markings cover, or have 

 once covered, a large part of the surface. Accompanying 

 these markings we find another, hardly less curious phe- 

 nomenon, the rounding off or planing down of the hardest 

 rocks to a smooth undulating surface. Hard crystalline 

 schists with their strata nearly vertical, and which one 

 would expect to find exposing jagged edges, are found 

 ground off to a perfectly smooth but never to a flat surface. 

 These rounded surfaces are found not only on single rocks 

 but over whole valleys and mountain sides, and form what 



