RELATION OF BIOLOGY TO GEOLOGICAL INVESTIGATION. 331 



salts would accumulate in its waters to such an extent and of such a 

 character as to modify or destroy its aquatic fauna. A reversal of such 

 conditions would change an inland sea to a lake, because soluble salts 

 would cease to accumulate in its Maters and the previous surplus would 

 gradually be removed by the free outflow which would result from such 

 a change. 



Inland seas may hold either more or less saline matter in solution 

 than does the ocean. In those which by reason of having no outlet 

 hold more, or hold a large proportion of other salts than sodium chlo- 

 ride, little or no gill-bearing animal life exists. Such seas may lie above 

 ocean level, as does Great Salt lake, in Utah, or below it, as does the 

 Dead sea. 



But the waters of some existing inland seas which have no outlet 

 have not yet become so impregnated with soluble salts as to destroy, 

 or to prevent the accession of aquatic life. This is at present the con- 

 dition of the Caspian sea, excepting some of its bays where from exces- 

 sive evaporation the water contains an excess of saline matter, but if 

 surrounding physical conditions continue the same as they now are the 

 present average degree of saltnessof this sea will continually increase. 

 Its surface being more than 80 feet below ocean level it can of course 

 have no outlet. The land area which is drained into it, being very 

 large there is a considerable and constant accession of saline matter 

 to its waters. Therefore it can not be doubted that if surrounding con- 

 ditions should remain unchanged the natural increase of soluble saline 

 matter will ultimately destroy all aquatic life in this sea. 



The Black sea is an example of an inland sea lying nearly at ocean 

 level, the difference between its level and that of the Mediterranean 

 and of the ocean really amounting to little or no more than the short 

 and slight river slope of the Bosphorus and of the Hellespont. The 

 proportion of soluble salts in the waters of this sea, like those of the 

 Caspian, Baltic, and other seas, differs greatly in different parts and at 

 different depths, the average proportion being less than that of the 

 oceans; but any cause which should diminish or increase its supply of 

 drainage water would increase or diminish its average saltness, as 

 already explained; and such increase or diminution would correspond- 

 ingly affect the character of its aquatic fauna. 



The general statements contained in the last three paragraphs con- 

 cerning the conditions which prevail in connection with existing lakes 

 and inland seas, and the circumstances upon which those conditions 

 depend, are introduced here for the purpose already indicated, and also 

 that they may be referred to in connection with the criteria which are 

 discussed in the following paragraphs: 



Because the waters in which even the latest of the sedimentary for- 

 mations of the earth were deposited have long ago passed away, and 

 their beds changed to dry land, the grounds upon which geologists 

 assume that of the formations which they have to investigate, some 



