CENOZOIC TIME — TERTIARY. ' 889 



bama Smith and Johnson have assigned the following thicknesses to its various sub- 

 divisions : Buhrstone, 300' ; Lisbon beds, 50' ; Ostrea sellceformis beds, about 65' ; in all 

 about 415'. 



4. The Claiborne was named by Conrad from Claiborne, Ala. The typical develop- 

 ment of this group is of very limited geographical extent, being confined to the drainage of 

 the Alabama and Tombigbee rivers (Langdon) ; but in Arkansas at White Bluff on the 

 Arkansas River and elsewhere, there are marly sands with a fauna showing Jackson affin- 

 ities, though they are at present classed as uppermost Claiborne. The typical Claiborne 

 bed is 16' thick ; the miite Bluff bed over it, 20'. 



5. The Jackson beds were so named by Conrad from typical exposures at Jackson, 

 Miss. They are sometimes improperly classed with the Vicksburg, under the name of 

 'iVhite Limestone. They occur on the Gulf slope east of the Sabine River. In Arkansas 

 and probably in Mississippi they extend some distance up the Mississippi Embayment, 

 overlapping Claiborne and Lignitic beds. They are clayey and lignitiferous in this region ; 

 but to the east, in Alabama, become calcareous and constitute beds of impure limestone. 

 Thickness over 50'. 



6. The Vicksburg, named by Conrad from typical exposures at Vicksburg, Miss. This 

 group is mainly composed of limestones, pure and impure, and like the Jackson is confined 

 to the Gulf slope east of Sabine River ; and unlike the preceding groups, it is little influ- 

 enced by the Mississippi Embayment. According to Langdon's figures its thickness varies 

 from 150' to 210'. The Bed Bluff group of Hilgard is scarcely separable faunally from this. 



General Bemarks. — Although it has been said that the Cretaceous (Chico) and the 

 Eocene (Tejon) deposits west of the Rocky intergrade without a perceptible break, 

 their respective faunas indicate that there is a break somewhere. On the Atlantic and 

 Gulf slopes there is abundant proof of a marked discordance, both faunal and stratigraphic, 

 between the Cretaceous and Eocene Tertiary series. In the Mississippi Embayment, at 

 least in eastern Arkansas, the earliest known Eocene beds pass up and over the Cretaceous, 

 while in southwest Arkansas, Texas, Alabama, and Georgia, broad areas of Cretaceous are 

 exposed ; in Maryland and Virginia, where lowest Eocene is wanting, Lignitic beds rest 

 upon the Cretaceous. 



II. Miocene and Pliocene, or Neocene of the Atlantic and Gulf 

 Borders. — While dredgings from the Grand Bank of Newfoundland, as well 

 as from St. George's Shoal, off the coast of Massachusetts, render it probable 

 that later Tertiary deposits exist beneath these shallow seas, the first distinct 

 exposures found on the Atlantic coast are those of Martha's Vineyard at Gay 

 Head and Chilmark, as recently proved through a study of the fossils by 

 Dall (1894). The next is near the village of Bridgeport in New Jersey. 

 These exhibit Miocene marls of black, yellow, and gray hues, with a thick- 

 ness of from 12 to 15 feet. The sands, clays, and marls from the Artesian 

 well at Atlantic City indicate that the thickness of the Miocene strata there 

 is not less than 700 feet. These deposits are mainly, if not exclusively, of 

 Upper or Yorktown Miocene age. 



In Maryland the escarpments along the western shore of Chesapeake Bay, 

 and along the Patuxent and Potomac rivers, show Miocene beds of sand and 

 clay, rarely indurated, and, at base, thick deposits of diatomaceous earth, 

 amounting in all to a thickness of 400 feet. In Virginia a similar series is 

 exhibited along the river courses ; and in the region of Dismal Swamp 

 younger beds of Pliocene age are reported. 



