LOWER CRETACEOUS FORMATIONS AND FAUNAS 585 
Some years before the investigation of the Texan Cretaceous 
was begun several geologists described strata in Virginia, Mary- 
land, Delaware, and New Jersey that are now usually referred to 
the Potomac formation, but they had few facts on which to base 
their age determination. Thus W. B. Rogers* described these 
beds as Upper Secondary, and provisionally referred them to the 
Upper Jurassic, though he later? suggested that they might form 
a ‘‘passage-group analogous to the Wealden of British geology.” 
Tyson also described a part of the same series under the desig- 
nation of ‘iron ore clays’’ which he at first referred to the Cre- 
taceous, but afterward‘ placed ‘‘at least as low as the Oolitic.” 
According to Dawson’ and Fontaine® Tyson considered that 
these beds belonged to the Wealden, but he seems not to have 
published that opinion. In 1886 Mr. W J McGee’ named and 
described the Potomac as a distinct formation including the 
above mentioned beds that had been discussed by Rogers and 
Tyson. Its determination as of Lower Cretaceous age has been 
mainly due to the paleobotanical work of Professors L. F. 
Ward? and W. M. Fontaine.? 
In 1869 Gabb and Whitney’ defined the Shasta group of 
California, stating that ‘It contains fossils seemingly represent- 
ing ages from the Gault to the Neocomian, inclusive.” The 
Shasta group has ever since been referred to the Lower Creta- 
ceous, and subsequent investigations have only confirmed the 
* Report of Prog. of Geol. Surv. of Va. for 1840, Richmond, 1841. Idem for 
1841, Richmond, 1842. Both reprinted in Geology of the Virginias, 1884, pp. 413- 
546. 
2 Proc. Bost. Soc. Nat. Hist., Vol. XVIII, 1875, p. 105. 
3 First Rept. State Agricultural Chemist of Md., pp. 41-43, Annapolis, 1860. 
4Second Rept. State. Agri. Chemist, p. 54, Annapolis, 1862. 
5 Trans. Roy. Soc. Canada, Vol. III, 1885, sec. 4, p. 18. 
6 Monograph U. S. Geol. Surv., No. 15, p. 5. 
7Rept. of Health Officer for the Dist. of Columbia for 1885, pp. 23-25; Am. 
Jour. Sci., 3d ser., Vol. XX XV, 1888, pp. 120-143. 
8 Am. Jour. Sci., 3d ser., Vol. XXXVI, 1888, pp. 119-131. 
9 Monograph 15, U.S. Geol. Surv., 1889. See also many subsequent articles by 
both authors and by McGkrE, DarToN, WHITE, MARSH, and CLARK listed in the 
accompany bibliography. 
to Paleeont. of California, Vol. II, p. xiv. 
