34 The Lower Cretaceous Deposits of Maryland 



Other accunnilations in water and on land are going on about us all 

 the time, and with those already described represent the formations of 

 Eecent time. 



HISTOEICAL REVIEW 



The more detailed and specific discussions of the Potomac Group and 

 its contained fossils, as is usually the case, were preceded by a long 

 period during which the geological and lithological relations chiefly at- 

 tracted attention, and this in turn was preceded by a still longer period 

 during which the subjects were still more general, and only passing 

 reference was made to the series of strata since designated the Potomac 

 Group, 



The earliest definite reference to rocks of this age is contained in 

 two papers by B. H. Latrobe, the first of which dates back to 1799 and 

 refers to the use of " Eappahannoc freestone " in the construction of the 

 lighthouse at Cape Henry. The second paper, published in 1809, de- 

 scribes this rock and its uses, and mentions the presence of the contained 

 wood and lignite from the vicinity of Fredericksburg, Virginia. 



The paper by John Finch, read before the Philadelphia Academy in 

 1823, and so often cited in historical discussions of Coastal Plain geology, 

 mentions the organictremains in the clay underlying the diluvial gravel 

 at Washington, and although not altogether unequivocal probably refers 

 to Potomac strata. Morton's paper of 1829, which was based on the 

 notes of Vanuxem, describes the lignite and charred wood of these rocks, 

 which they include in their " Secondary formation." 



The first intimation of the wide extent of the Potomac formations is 

 contained in an early report of Edward Hitchcock, published in 1833, in 

 which he mentions the probable distribution of deposits of this age from 

 Cape Cod to the Gulf of Mexico. 



In a paper published by Thomas G. Clemson in 1835 there is a good 

 description of the Potomac material near Fredericksburg with its fossil 

 wood and lignites, and with the first reference to impressions of plants 

 which he says are finely preserved in blue argillaceous fissile beds from 

 six inches to a foot in thickness. 



