592 TIMOTHY WILLIAM STANTON 
ever these questions may be finally decided, it is evident that the 
discussion as to the Potomac formation is not so much on its 
correlation with deposits elsewhere as on the more general 
question of the upper limits of the Jurassic. 
The Tuscaloosa formation The beds known under this name 
have their principal development in Alabama, extending thence 
eastward into Georgia and westward into Mississippi. According 
to Prof. E. A. Smith* the formation consists of ‘‘heavy bedded 
purple and mottled and gray clays in the lower parts, alternating 
with more distinctly stratified clays containing an abundance of 
plant remains, chiefly in the form of leaf impressions. Above 
these clayey beds are sands of various colors, white, yellow, gray, 
pink, and purple, usually micaceous and strongly cross-bedded. 
In many places irregular pockets of small angular chert pebbles 
are interbedded with the sands, but these pebble beds make only 
a very small proportion of the strata. In places also beds of 
dark red and mottled clay occur in the upper part of the forma- 
tion.” In the eastern part of the area it rests on ancient crystal- 
line rocks while farther west it laps up on the Paleozoic sed- 
iments. The overlying beds are of Upper Cretaceous age. 
The thickness of the Tuscaloosa is estimated at 1000 feet. 
Lithologically and stratigraphically the Tuscaloosa is seen to 
correspond closely with the Potomac and the evidence of the 
flora leads to the same correlation. Professor Smith? publishes 
a list of 35 species of fossil plants determined by Professor Ward, 
who compares them with the Amboy Clay (2. e. Uppermost Poto- 
mac) flora. In later publications Professor Ward3 definitely 
correlates the Tuscaloosa with the Amboy and Raritan clays, 
suggesting that possibly one of the older horizons of the Poto- 
mac may also be represented in Alabama. No animal remains 
Protessor Ward obtained plants that he regards as Lower Cretaceous, and the Atlanto- 
saurus fauna has also been found in this same region, but what relation the plant-beds 
have to the vertebrate horizon, and whether the plants and vertebrates do not really 
occur in the same bed has not been determined. ; 
* Rep. on Geol. of the Coastal Plain of Ala., pp. 307-308, Montgomery, 1894. 
2 [bid., p. 348. 
315th Ann. Rept. U.S. Geol. Surv., pp. 337-338; 16th Ann. Rept., p. 470. 
