c\ CORRELATION OF MARYLAND MIOCENE 



adoptoil for species by the autliors of this part of the volume, as com- 

 pared with the views prevalent in the time of Lyell. However, about 

 13 per cent of the New Jersey species survive, and 1-i per cent of the 

 Floridian Chesapeake, so the estimate is not far from normal for the 

 lower American jMiocene. For the upper Miocene of Duplin about 20 

 per cent are estimated to survive, and 19 per cent in the Suffolk district 

 of Virginia. The intermediate Yorktown beds have about 17 per cent 

 of survivors. 



T have already called attention to the fact that the Miocene of South 

 Europe is of a more tropical character than that of our typical Chesa- 

 peake, and that a more appropriate comparison in detail may be had 

 with the Miocene of Northern Europe, Belgium, North Germany and 

 Denmark. Even the latter is less boreal or apparently lived in warmer 

 waters than the species of the Maryland beds. It would seem that, in 

 America, the change at the end of the Miocene was marked by a slight 

 elevation and a distinctly warmer water fauna which pushed its way 

 northward at least as far as Virginia, and possibly to Martha's Vine- 

 yard, where the genus Corhkula, a distinctly southern form, has been 

 detected. From a survey of the available literature it would seem, how- 

 ever, that, on the continent of Europe, the Pliocene fauna which made 

 its way southward was of a somewhat more northern type than the 

 Miocene which it succeedoil. D' a change in the ocean currents corre- 

 sponding to our present Gulf Stream, took place at the end of the ^Vfio- 

 cene, by which the tropical waters were directed ovei- a longer extent 

 of the Atlantic coast than was the case during the Chesapeake epoch, 

 and hence became more or less cooled oft' before making the transit of 

 the North Atlantic, the temperature conditions necessary to account for 

 this difference in the faunas, would have been provided. 



Of live liundred species of gastropods enumerated by Hoernes from 

 the \'ienna l)asin 20.6 per cent are regarded as surviving to the present 

 epoch : a number without doubt too great from the standpoint of the 

 average modern estimate of what constitutes a species. But it would 

 carry us too far to attempt to rectify this estimate in detail. 



In the work of Nyst (1843) on the Tertiary of Belgium the Diestien 

 of Dumont and the fauna of the Bolderberg were referred to the Pliocene 



