260 GEOLOGY. 



appear that the sub-Miocene surface had been deeply eroded before the 

 deposition of the Miocene beds. The slight erosion was probably the 



result of low altitude, rather than of a short period of exposure, for a 

 considerable interval of time seems to have elapsed between the deposi- 

 tion of the Eocene and that of the Miocene of this province. 



The northernmost exposure of the Miocene on the Atlantic coast is 

 on Martha's Vineyard. Between this point and Georgia it appears at 

 the surface interruptedly (Fig. 442). From New Jersey to North Car- 

 olina it fails only about the principal bays, where younger formations 

 conceal it. In its surface distribution it sustains the same relation to 

 the Eocene that the latter does *to the Cretaceous, though it sometimes 

 overlaps the Eocene, completely concealing it. Like the other forma- 

 tions of the Coastal plain, the Miocene beds dip seaward and are con- 

 cealed by younger beds before the present shore line is reached. The 

 general relations are indicated by Fig. 380. Even in the belt where the 

 Miocene is mapped as appearing at the surface, it is often thinly covered 

 with younger deposits. The series originally extended inland far be- 

 yond its present border, as shown by numerous outliers. In New 

 Jersey, 1 the Miocene series reaches a thickness of 700 feet; in Mary- 

 land, 2 about 400 feet, and in North Carolina still less. 



The Miocene of the Atlantic coast is for the most part made up of 

 unconsolidated beds of sand, clay, and shell marl. In places, diatoma- 

 ceous earths (variously known as Richmond earth [from Richmond, 

 Va.], Bermuda earth, Tripoli, infusorial earth, etc.) are found in beds 

 of such thickness (30 or 40 feet 3 ) as to be commercially valuable. 



Much of the Miocene sand is remarkable for its even grain. It is 

 often aluminous, and has a remarkably soft feel, which has been de- 

 scribed as " fluffy. ?; It is often beautifully mottled with delicate colors, 

 and in many places contains small but beautifully smoothed quartz 

 pebbles. Locally, it is cemented into sandstone, and rarely the cemen- 

 tation has gone so far as to convert the sandstone into quartzite. 



The Miocene beds of the Atlantic coast are generally grouped under 

 the name Chesapeake (or Yorktown). They were formerly regarded as 

 Upper Miocene, but the present tendency is to restrict the term Mio- 

 cene to the Chesapeake, the former Lower Miocene being classed as 



1 Reports of the State Geologist of New Jersey, especially Report of 1892 (Clark) 



2 Clark, Maryland Geol. Surw, Vol. I; also volume on the Miocene, 1904. 



3 Maryland Geol.. Surv., vol. on Miocene, p. xxx. 



