Physical Geography. 107 



waters ; and though these facts bear but slightly on my 

 present subject, seals being air-breathing Mammalia, 

 yet in some of the lakes of Sweden ordinary marine 

 Crustacea are found. This may be accounted for in 

 the way that I now attempt to account for similar 

 peculiarities in the Old Ked Sandstone strata. These 

 Swedish lake-areas were submerged after the close of 

 the Glacial epoch ; and being deep basins (scooped out 

 in a manner which I shall explain in a later chapter), 

 while the land was emerging by upheaval, and after 

 its final emergence, the salt water of the lakes fresh- 

 ened so slowly, by influx of rivers, that some of the 

 creatures inhabiting it had time by degrees to adjust 

 themselves to new and abnormal conditions. * 



Again, we may suppose a set of circumstances such 

 as the following : If, by changes of physical geography 

 of a continental kind, a portion of the Silurian sea got 

 separated from the main ocean, more or less like the 

 Caspian and the Black Sea, then the ordinary marine 

 conditions of the ' passage beds,' accompanied by some 

 of the life of the period, might be maintained for what, 

 in common language, seems to us a long time. The 

 Black Sea was once united to the Caspian, and the 

 Caspian to the Aral, forming one great inland sea, which 

 under varying physical conditions, has more than once 

 changed its form and extent. At all events, since its 

 separation from the Black Sea the Caspian has been 

 simply a great brackish lake. The Black Sea is now 

 steadily freshening ; and it is easy to conceive that by 

 a geographical change, such as the upheaval of the Bos- 



1 For much valuable information on this subject, see 'Annals 

 and Mag. of Nat. Hist.' third series, vol. i. 1858, p. 50, 'On the 

 Occurrence of Marine Animal Forms in Fresh Water,' by Dr. E. von 

 Martens : translated by Mr. W. S. Dallas. 



