40 The Lower Cretaceous Deposits op Maryland 



contributed by W J McGee to the Eeport of the Health Officer of the 

 District of Columbia for 1885. In 1887 Professor Fontaine submitted 

 a paper embodying his results to the American Association for the Ad- 

 vancement of Science, a brief abstract of which was published in 1888, 

 from which the following is quoted : " The name Potomac formation 

 has been applied to a series of newer Mesozoic sands, gravels, and clays, 

 sometimes cemented into sandstones and conglomerates, which appear 

 along the inner margin of the Coastal Plain, forming the basal member 

 of the undisturbed Mesozoic and Cenozoic formations of the eastern 

 United States, in Virginia, Maryland, Delaware, and perhaps other 

 States. It comprises two members — an upper, consisting generally of 

 variegated clays which are well exposed about Baltimore, and a lower, 

 consisting predominantly of sands and gravels, well exposed in the bluffs 

 of the Potomac Eiver below Washington. The upper member is known 

 only north of Fredericksburg, and the lower is best developed from 

 Washington to Richmond. 



" The age of the formation, as indicated by its flora, appears to coin- 

 cide approximately with that of the Lower and Middle ISTeocomian [mis- 

 printed Neuronian] of Qreenland and Europe." 



It was in December, 1887, that Mr. J. B. Hatcher, under instructions 

 from Professor 0. C. Marsh, collected a considerable number of verte- 

 brate bones from an iron mine near Muirkirk, Md. He also found in 

 the same beds some small cones representing the genus Sequoia, and 

 much silicified wood and lignite. The bones were described by Professor 

 Marsh and the results published at once. As to the geological signifi- 

 cance of these forms, Professor Marsh says : 



" The fossils here described, and others from the same horizon, seem to 

 prove conclusively that the Potomac formation in its typical localities 

 in Maryland is of Jurassic age, and lacustrine origin. There is evidence 

 that some of the supposed northern extensions of this formation, even if 

 of the same age, are of marine or estuary origin.'^ 



The next year Professor Uhler read a paper before the American 

 Philosophical Society in which the name Baltimorean was proposed for 

 the lower beds and Albirupean for the upper, which, however, included 



