THE ORIENTAL MIGRATION. 277 



fact that in the upper strata of the enormously thick 

 Sicilian pliocene deposits are found a number of 

 arctic or subarctic species of mollusca which are 

 entirely foreign to the Mediterranean fauna. It is 

 generally supposed that these reached the Mediter- 

 ranean area by the newly opened Straits of Gibraltar 

 in later pliocene times, and that the lower Sicilian 

 deposits must therefore have been laid down earlier. 

 So far the deductions are perfectly correct, if we assume 

 the northern mollusca to have arrived in the Atlantic 

 at the time stated. However, they must have reached 

 the Atlantic much later not till pleistocene times 

 if we adopt the above-stated suggestions as to the 

 age of the Forest-Bed (cf. p. 125). Moreover, the 

 great similarity between the faunas of Southern 

 Spain and North-western Africa indicate that the 

 formation of the Straits of Gibraltar is of very 

 recent date. The northern mollusca, of course, 

 could not have reached Sicily till later. To suppose 

 that the Sicilian deposits have been uplifted 7000 feet 

 since then is no doubt contrary to all our geological 

 teaching, but we must remember that this is altogether 

 an exceptional case. The area in question has prob- 

 ably ever since been in the immediate neighbourhood 

 of an active volcano, and the rate of the uplift has 

 therefore been immeasurably greater than at other 

 localities with which this one might be compared. The 

 disconnection between Tunis, Sicily, and Southern 

 Italy was evidently produced by a subsidence of the 

 tract of land uniting these countries. If we suppose 



