198 Kepoet op the State Geologist. 



deposits is" so clear, that hy studying the plants and insects, we 

 can form an idea of the alternation of the seasons. Again, we 

 may cite the lakes of Beauce and of Limagne during the Oligo- 

 cene, those of (Eningen and of Radoboj daring the Miocene, etc. 



Brackish water facies. — The same phenomenon of transporta- 

 tion, the effects of which have just been mentioned, has often 

 resulted in the mingling of the terrestrial and fresh water fauna 

 and flora with the salt water fauna. This mingling is one of the 

 essential characteristics of the facies of estuaries and lagoons. 

 Everywhere at the mouth of rivers, and wherever marshy lands 

 or lagoons border the sea, shells, bones and vegetable remains 

 even though not inhabitants of those localities, have accumu- 

 lated and been fossilized. These places have generally a special 

 fauna which we call a brackish water fauna, consisting of 

 species which are able to bear great variations in the saltness 

 of the water, and to exist in a muddy medium. Yarious 

 forms of Cardium, Mya, Gyrene, OeritMum, particularly 

 those of the genus Potamides, and Crustaceans near to Sphceroma, 

 are the principal types of this brackish water fauna. In these 

 localities are often found skeletons of mammals and reptiles, 

 sometimes also of birds. The return of the sea from time to 

 time brings marine forms whether or not these could live 

 in such localities, while the descending rivers drag down the 

 remains of the fresh water and terrestrial flora and fauna. From 

 these changes result frequent alterations in the character of the 

 deposits which, moreover, were accumulated with great rapidity. 



As an example we may cite the Franco-Belgic coal basin, 

 where the coal beds with the fresh water bivalve genus 

 Anthracosia, olten alternating with marine sediments, indicate 

 the frequent return of the sea in the lagoons where the vegetable 

 remains accumulated. 



The London clay, the Upper Eocene of the Paris basin, the 

 Oligocene and Miocene south of Bordeaux, etc., also show char- 

 acteristics of these brackish water or estuary deposits. 



In the Sarmatio beds, which in France are referred to the 

 Upper Miocene, we find the best type of brackish water 

 deposits. At that epoch a vast sea extended from the neighbor- 

 hood of Vienna to Turkestan. The Black sea, the Caspian sea, 

 the sea of Aral, are the feeble remains of that immense sheet of 

 water. The saltness seems to have undergone considerable vari- 

 ation ; it may have varied also in different parts, and numerous 

 lagoons must have occupied the borders of this interior sea. 



