LOWER CRETACEOUS FORMATIONS AND FAUNAS 587 
From this review it is seen that the Lower Cretaceous forma- 
tions now known in the United States are the Comanche series 
of the Texan region, the Shasta group (including Knoxville and 
Horsetown beds) of the Pacific Coast, the Kootanie of Montana 
and possibly of the Black Hills, the Potomac of the Atlantic 
coastal plain, and the Tuscaloosa of the Gulf border. Only the 
first two are marine formations. In our comparisons of these 
formations it will not be necessary to enter into minute details 
of stratigraphy and lithology, since the most general descrip- 
tions will show that in most cases we have to deal with contrasts 
rather than with resemblances. This is true not only when the 
marine beds are compared with the non-marine, but also when 
the two marine formations are compared with each other, or the 
Potomac is compared with the Kootanie. In the descriptions 
that follow, mainly summarized from the latest published 
accounts, the principal characteristics of each formation are 
given, beginning with the fresh-water beds. The statements 
concerning the invertebrate faunas embody more of the results 
of my own studies. 
GEOLOGIC DESCRIPTION OF THE FORMATIONS. 
The Potomac formation.— This term was originally applied by 
McGee to certain non-marine beds in Maryland, the District of 
Columbia, and Virginia, resting against the old crystalline rocks 
of the Piedmont region, and unconformably overlain by marine 
Upper Cretaceous deposits. The Potomac as thus defined is 
composed of irregular deposits of variegated clays, sand, arkose, 
pebbles, and bowlders, with local lenses of iron ore and lignitic 
seams. The sand and arkose are sometimes indurated, but fre- 
quently are unconsolidated deposits. In general, the arenaceous 
deposits seem to predominate in the lower part of the series 
and argillaceous beds in the upper, though no single stratum 
retains the same lithologic character over any considerable 
area. The estimates of thickness vary from 500 or 600 feet 
(McGee) to 1175 feet (Ward). Professor Ward,’ who has 
*See 15th Ann. Rept. U. S. Geol. Surv., pp. 313-397, and 16th idem, pp. 469-540. 
