89 



until it finally disappears below the level of the tide near the mouth 

 of Chingoteague creek on the Rappahannock, and Mathias's Point 

 on the Potomac. The Miocene spreads eastward from the line con- 

 necting these two localities to the termination of the peninsula ; while 

 some of its lower beds extend west of the same line into the Eocene 

 district, where they are confined, however, to the highest portions of 

 the land. 



After offering numerous details relating to the range and limits of 

 these two divisions of the Tertiary Deposites, the paper treats in the 

 next place of the arrangement and composition of the Miocene strata, 

 which are shown to possess a close general analogy in these respects 

 to the Miocene beds of the peninsula of the York and James rivers, 

 described in a former communication. The two most interesting 

 points of agreement are the occurrence of the blue marls low down in 

 the series, and the presence of the thin band of ferruginous rock sepa- 

 rating the Miocene from the overlying diluvium. 



In general the blue marl at the base of the Miocene, is the most 

 replete in fossils, though towards the eastern extremity of the penin- 

 sula, shells, &c, abound in the upper sands and clays. Usually the 

 tipper beds of the Miocene in this district are destitute of fossils, 

 though full of their casts and impressions. 



These strata consist generally of light coloured sandy clays, distin- 

 guished by a sulphurous smell, and an acid and styptic flavour. 

 Carbonate of lime is not abundant, but the sulphate of lime occurs 

 sometimes in valuable proportion. Sulphate of iron, sulphate of 

 alumina, free sulphuric acid, sulphur, and even an appreciable amount 

 of sulphate of magnesia are also met with. 



The fossil impressions in these beds are beautifully distinct, and 

 appertain to all the species of shells which are found in perfect condi- 

 tion in the subjacent strata. In the blue clayey marl beneath, there 

 often occurs a notable proportion of green sand, which is also found 

 in some of the other Miocene strata, mixed pretty largely with com- 

 mon sand and clay, in beds destitute of fossils. 



The paper treats in detail of many of the more interesting localities 

 in the Miocene district, describing the stratification, and presenting 

 evidence of the relative fertilizing agency of the several beds. 



The fossil species which characterize the Miocene strata, are next 

 enumerated. 



In the next section, an account is given of the arrangement and 

 composition of the Eocene strata of the peninsula. 



