24 J. L. Campbell—Silurian Formation in Virginia. 
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a 
lic beds (a), as originally deposited on the ancient sea-bottom, — 
underlie those of 6, while these again were overlaid by the 
ed nly occasional fucoid plants, and brachiopod mol- 
this formation, gives a dark brown color to the soils produced 
by its disintegration. These are among the best and most 
tegrating agencies, as to be left as a covering on the faces of 
many of the limestone hills throughout a large extent of the | 
Great Valley. This chert bed varies in thickness from one to 
ten feet within the range of a few miles; but it and the brown 
sandstone lower down serve as well defined land-marks for this 
whole formation. ‘The brown sandstone has preserved imper- 
fect impressions of several species of ——Te shells, while 
in the chert bed are found in some localities large numbers | 
silicified shells of gasteropod and cephalopod mollusks. This 
division (c) by disintegration yields light clay and sandy or 
pebbly soils, according to the varying characters of the out- _ 
cropping strata. These soils are only moderately productive— __ 
some of them very poor. deposits of limonite ore in 
_ this formation have been mined in past years to supply some 
_ of the iron furnaces in Augusta county. | : a 
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