THE REVOLUTIONS OF THE CRUST OF THE EARTH. 329 



animals is so limited that they are born and perish in a period which 

 for the Testacea is recent.* 



The least complicated seem best to resist the changes of condition due 

 to the influence of the revolutions of the terrestrial crust ; those which 

 through simplicity of elementary constitution approach nearest the min- 

 eral kingdom share somewhat its resistance to the agents of destruction. 



The presence of the fossil remains of marine animals in the interior of 

 continents proves that the configuration of the latter is not invariable. 

 We have already spoken of the slow rising of Australia. Central Asia, 

 also, must be the elevated bed of a former ocean, for Kirghiz and China 

 show that there must have been an interior sea where now exists the 

 desert of Gobi.i Besides, deposits of shells have been observed at ele- 

 vations of from three to four hundred yards, and even higher.J The 

 levels of erosion of the Alps, observed by Sharp at heights of 9,000, 

 7,500, and 4,800 feet, prove that tbe level of the sea has changed several 

 times.§ In former periods England was united to the continent, as is 

 evident from the similarity of the molluscal fauna of the British Isles 

 with that of the continent, and from certain geological considerations 

 in regard to the fauna of the Quaternary Mammalia. Thus we know of 

 no species of fluvial Testacea in England which is not found upon the 

 continent, while several species of recent appearance on the continent 

 are wanting in England. If the Azores and the Canaries possess species 

 peculiar to them, this proves that by their isolation these islands have 

 become centers of distinct creations, which is not the case with the Brit- 

 ish Islands, on account of their dependence upon the coutiuent. 



M. Laurent, in charge of tbe formation of artesian wells in the north- 

 ern part of Sahara, asserts in his report that the desert was once a sea, 

 communicating through the gulf of Gabes with the ocean. Among the 

 fossils discovered, oue, the Oardium eclide, is frequently found in the Med- 

 iterranean, and still lives in the salt lakes of the desert. 



The existing continental fauna of Morocco, and of Algeria as far as 

 Bacca, is almost identical with the fauna of Southern Europe. Only 

 from the Senegal to the Nile commences the fauna of true African char- 

 acter. The elephant, the rhinoceros, the hippopotamus, the giraffe, the 

 crocodile are not found beyond the desert, and the continental fauna of 

 Africa coincides closely with that of Morocco, while the latter has much 

 analogy with the European fauna. The Innuus ecaudatus is found upon 

 the rocks of Gibraltar. The Sorex etruscus of Italy re-appears in Algeria. 

 Strauch shows the identity of the reptiles of the two coasts, and Eric- 

 son affirms that a large number of insects of Central Europe and a still 

 greater number of the Mediterranean countries are the same as those of 

 Morocco and of Algeria, while there are very few species identical with 



* Lyell, Elements of Geology, trad. Gineston, t. i, p. 213. 

 t Humboldt, Asie Central, t. ii, p. 138. 

 t D'Archiac, Histoire des Progres de la Gtologie, t. ii, p. 302. 

 § Quart. Journ. Geolog. Soc., 1855, t. xii,4G, pp. 102-123. 



