Description of the Region. 



Geographic Position. — The region discussed in this paper comprises a thou- 

 sand square miles around Harper's Ferry, about equally divided between 

 Virginia, West Virginia and Maryland. 



Topography and general Geology. — The topographic features of the region 

 are, briefly, two mountain lines and three valleys. In order, from west to 

 east, they are Shenandoah valley. Blue ridge, Middletown valley, Catoctin 

 mountain, and Leesburg valley. The position of the ridges is shown on the 

 geologic map by the black areas, the sandstones and the ridges being nearly 

 everywhere coincident. 



The last valley is composed of Mesozoic rocks and does not concern the 

 question under discussion ; the middle or Middletown valley is composed of 

 schists, injected with granite dikes, and will only be alluded to briefly ; the 

 first or Shenandoah valley is occupied by the Potomac and Shenandoah 

 rivers, and is almost entirely floored by the lower Silurian limestone. 



Of the two ridge lines, Catoctin mountain on the east is a single ridge and 

 for the most part formed of sandstone. Over its southern third the summit 

 is made of epidotic schists, and the sandstone retreats to the eastern side. 

 The other mountain line, the Blue ridge, is single more in name than in fact, 

 and usually is a double ridge, with summits of sandstone. This sandstone, 

 like that of Catoctin, leaves the summit to the epidotic schist in its southern 

 part and retreats to the western slope. The main line of what is called Blue 

 ridge in Virginia here laps past the South mountain of Pennsylvania and 

 Maryland. After paralleling each other for twenty miles, each dies away, 

 while the name ("Blue ridge") jumps from one to the other. 



West of South mountain and the Blue ridge there is a series of lesser 

 knobs and short ridges in a fairly continuous line. These, like the main 

 ridges, are capped with sandstone. Between the sandstones of the main and 

 lesser ridges and the valley limestones there are calcareous and argillaceous 

 shales. Between the Blue ridge and South mountain there are schists con- 

 taining eruptive granite, just as in the Middletown valley east of South 

 mountain. 



Surveys in the Region. 



The problem of the geologic age of the belt of rocks upon which Harper's 

 Ferry is situated was first approached by the U. S. Geological Survey in 

 1883. Then the senior author examined the Blue ridge at Balcony falls and 

 in the adjacent country. In subsequent years additional information was 

 gathered at many points by him, and during the last summer by the junior 



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