ABSTRACTS 759 



Arthur Keith, geologist; one page of columnar section, a topographic 

 map (scale i: 125000), a sheet showing theareal geology of the district; 

 another showing the economic geology, and a third exhibiting structure 



sections. 



Th* folio describes that portion of the Appalachian province which 

 is situated between parallels 39° and 39° 3°' and meridians 77° 3°. 

 and 78°. The tract contains about 950 square miles, and falls within 

 Washington and Frederick counties, Maryland ; Loudoun and Fauquier 

 counties, Virginia; and Jefferson county. West Virginia. 



The folio begins with a general description of the province, which 

 shows the relation of the Harper's Ferry tract to the whole. The local 

 features of the drainage by the Potomac and Shenandoah Rivers and 

 their tributaries (Goose, Antietam, and Catoctin Creeks) are treated. 

 The various forms of the surface are pointed out, such as Shenandoah 

 Valley, Blue Ridge, and Catoctin Mountain, and their relations to the 

 underlying rocks are made clear. 



Under the heading Stratigraphy the geologic history of the Appa- 

 lachian province is presented in outline, and the local rock groups are 

 fully described in regard to composition, thickness, location, varieties, 

 and mode of deposition. 



The formations range in age from Algonkian to Cretaceous, the 

 greater portion being Algonkian, Cambrian and Silurian. The Silurian 

 rocks appear in Shenandoah Valley, the Cambrian in Catoctin Moun- 

 tain and Blue Ridge, the Algonkian between these ridges, and the 

 Juratrias east of Catoctin. The Algonkian rocks are chiefly granite 

 and epidotic schist ; the Cambrian rocks, sandstones and shales, passing 

 up into limestones; the Silurian rocks, limestones and shales; and the 

 Juratrias rocks, red sandstone and shale and limestone conglomerate. 

 The details of the strata are shown in the columnar section. The 

 manner in which each kind of rock decays is discussed, and how the 

 residual soils and forms of surface depend on the nature of the under- 

 lying rock. 



In the discussion of Structure, after a general statement of the 

 broader structural features of the province, three methods are shown m 

 which the rocks have been deformed. Of these the extreme Appa- 

 lachian folding is the chief ; next is that developed in the Juratrias rocks; 

 and least in importance are the broad vertical uplifts. Three degrees 

 of extreme deformation appear in the Paleozoic rocks — folding, fault- 

 ing, and metamorphism — each being best developed in a certain kind 



