TERTIARY FORMATION. 



269 



the newer and older pliocene, the miocene, and eo- 

 cene periods. 



The newer Pliocene, — Mr. Conrad has pointed out 

 a distinct deposite of this formation in Maryland. It 

 is well characterized about three miles north of the 

 low sandy point which forms the southern extrem- 

 ity of Xhe peninsula near the mouth of the Potomac. 

 Here the bank rises to an elevation of some fifteen 

 feet at its highest point, and the fossils are visible 

 to the extent of a quarter of a mile. The stratum 

 consists of sand and clay, disposed in horizontal 

 beds, with an abundance of shells similar to exist- 

 ing species. The distance from the nearest point 

 on the Atlantic Ocean is about 45 miles, but nearly 

 100 miles by the course of the bay. The extent of 

 this formation is not known. 



Older Pliocene and Miocene Formations. — These 

 tertiary beds show themselves in a wide and, at 

 present, an undefined belt in the southern extrem- 

 ity of New-Jersey, and extend through Delaware, 

 Maryland, Virginia, and North Carolina, in the 

 southern part of which last state, and in part of 

 South Carolina, they only occur in interrupted 

 patches, thinning out and disappearing altogether 

 after reaching the Santee River in South Carolina. 

 In New-Jersey, Maryland, and Virginia, the pro- 

 portion of recent to extinct species among the fos- 

 sils hitherto discovered does not, in the average, ex- 

 ceed 20 to 25 per centum, which, therefore, places 

 their origin in the miocene era. While the fossils 

 south of Virginia show that the southern portions 

 of the tertiary deposites belong to the older pliocene 

 era. South of Virginia the tertiary beds contain 

 nearly two thirds recent species, while north of that 

 limit they contain a less proportion of living species 

 than one fifth. The total number of species of 

 shells found in our miocene formation is about 200, 

 40 of which only are living shells., and inhabit the 

 adjacent coast. 



