288 THE TRANSFORMATIONS OF THE ANIMAL WORLD 



trary, the shore has undergone a retreat of the 

 sea which has facilitated at that point the establish- 

 ment of a world of lagoons or estuaries, we see 

 brackish deposits with a special fauna of Cyrenas, 

 Potamidse, and Melanias superpose themselves on 

 the thoroughly marine sands of the earlier period. 

 These are constant facts in the history of all seas 

 on this earth ; and it might even be said that the 

 geological history of each region of the globe is 

 nothing but a long alternation of those oscillations 

 of the bottom of the sea which, at the present time, 

 manifest themselves in repeated superpositions of 

 the strata termed heteropic, that is to say, of diverse 

 nature and fades, at once lithological and faunic. 

 I will make these facts more precise by a few ex- 

 amples. 



The history of the Pliocene period in the basin of 

 the Rhone, in Italy, in Spain, and, more generally, 

 in the whole Mediterranean basin, comprises the 

 following series of episodes. To begin with, a 

 period of great submarine subsidences, accompanied 

 by phenomena of the deep scooping out of all the 

 continental valleys. After this, the sea penetrates, 

 by progressive incursions, into these deep and nar- 

 row valleys : we first observe, at the bottom of the 

 Pliocene deposits, layers of the fauna of brackish 

 waters (Strata of Congeries) indicating a first stage 

 of lagoons ; then the incursion becomes quicker, 

 and at a depth of several hundred metres under 

 water there is formed a blue slime, characterized by 

 certain species of smooth Pectinidae, Dentals, 

 Pleurotomas, etc. ; this is the Plaisantian stage. 

 After this, the sea tends to retreat by degrees to- 



