332 THE REVOLUTIONS OF THE CRUST OF THE EARTH. 



we find for the first time numerous scratches whose origin, much dis- 

 cussed, has been recently attributed to the action of ice. A similar and 

 very pronounced arrest of vegetation is manifest after the Jurassic for- 

 mation, in the Cretaceous deposit, and at several epochs during the 

 Tertiary period, which is especially demonstrated by the predominance 

 in Europe now of tropical, now of hyperborean types. A striking ex- 

 ample of a notable change of climate is given by E. Eorbes. This 

 savau has registered a list of fifty species of shell- fish which inhabited 

 the British seas at the time of the formation of the coralline crag 

 and of the red crag (of the Pliocene). All these mollusks are recent, but 

 they are completely wanting in the Pleistocene or glacial deposits of 

 England. We find them in those of Sicily, of Southern Italy, and of the 

 Grecian archipelago. E. Forbes thinks that these mollusks resorted 

 for a time to these shores, where they found conditions favorable to 

 their existence, and after the amelioration of the climate returned to 

 the British seas.* 



Mr. Heer has drawn attention to the fact that we find in many local- 

 ities of France and of Germany bones of the marmot and of the rein- 

 deer, which can only be accounted for by a lowering of the temperature 

 of Central Europe, which permitted these animals to live in latitudes 

 less elevated and at a less vertical height. M. Lartet has mentioned in 

 a letter addressed to the Academy of Sciences the presence of the 

 Ovihus moschatus in several localities in the ancient province of Peri- 

 gord. In the Quaternary then it must have lived lo'^ farther south, for 

 at the present period it first existed only in ]!Torth America and never 

 jiassed beyond the sixtieth degree. M. Alph. Milne Edwards found in 

 the bone-caves, among the animal remains, portions of the large owl 

 {Stryx nyctea) which to day inhabits the polar regions.t 



The glaciers, whose growing proportions we have mentioned, were 

 once much more extensive than they now are, as the moraines prove 

 we find in Piedmont and Lombardy, upon the Jura, and upon many other 

 mountain chains, where now no glaciers exist. We find these moraines 

 in the Vosges, in the Black Forest, in the mountains of Scotland, upon 

 the Apennines, Mount Libanus, the Alleghanies in Northern America, 

 in Xew Grenada, and in New Zealand.f It is well known that the 

 glaciers move along the sides of the mountain more or less rapidly in 

 proportion to the slope. They would end then by overflowing the 

 entire valley if the melting of their extremities did not keep them at a 

 certain height. It is only after a series of exceptionally cold years that 

 any progress can be perceived in the glaciers, while only a few very 

 warm years sufflce to produce a very sensible retreat. Now, if in 

 former times the glaciers extended to the environs of Turin, this would 



prove that this extension was due entirely to a period of severe win- 



^ ^p 



* Lyell, Elements of Geology, Gth edit., t. i, p. 330. 

 t Favre, Eech. G4ol., t. i, p. 182-lti3. 

 tibid,, p. 181. 



