MARYLAND GEOLOGICAL SUKVEY 27 



Tyson, State Agricultural Chemist, in his two reports to the House 

 of Delegates in 1860 and 1862, described the mineral resources of 

 Prince George's County, mentioning particularly the deposits of iron 

 ores, marls, and tripoli earth. 



In 1892 Clark, in his article on ''The Surface Configuration of 

 Maryland," gave many facts pertaining to the topography of Prince 

 George's County. In the following year Williams and Clark brought 

 together in the volume "Maryland, Its Kesources, Industries and 

 Institutions," all that was then known in regard to the physical 

 features, geology and mineral resources of the State, while in 1897 

 Clark, in Volume I of the Maryland Geological Survey, contributed 

 a more detailed report on the same subjects. These reports contain 

 brief descriptions of all the geological formations of the County then 

 recognized, and much information regarding the physical features 

 and economic resources. 



Another important publication to be mentioned is "The Washing- 

 ton Folio" of the United States Geological Survey by Darton and 

 Keith. The area described includes all of the District of Columbia 

 and a large part of Prince George's County and was the most com- 

 plete work on the geology of the region published up to that time. 

 Each of the geological formations, differentiated, is described and 

 its relation to the other formations discussed. 



A similar work, "The Patuxent Folio," by Shattuck, Miller and 

 Bibbins, published in 1907 by the United States Geological Survey, 

 contains more detailed information of the physiography, geology, and 

 mineral resources of the region than had previously been brought 

 together in one publication. The area covered by the Patuxent folio 

 includes a portion of the District of Columbia and all of Prince 

 George's County except small areas in the northern and southwestern 

 portions. The classification of the formations there used is the same 

 as that recognized in this report. 



HISTORICAL REVIEW OF THE VARIOUS GEOLOGIC GROUPS. 



The Crystalline Roclcs of the Piedmont Plateau. — Until within 

 recent years geologic literature contained very few and brief descrip- 

 tions of the crystalline rocks of this region, and at the present time 



