GEOLOGY OF GILES COUNTY, VIRGINIA 
367 
and redeposition by circulating ground waters as the rocks 
weathered. 
The manganese ore of the Shenandoah was probably widely 
scattered in the basal portions, originally as a carbonate. Here 
it was dissolved by ground waters. Solution channels, caving, 
or folding caused brecciated zones, in which the ore was rede- 
posited in a concentrated form. The interstices were thus filled 
with ore, and the silica of the chert was in some cases partially 
replaced by the manganese oxide. 
In a similar manner, the ore which is found in the Rockwood 
and Giles formations was originally disseminated in the basal 
Giles. The manganese was concentrated by descending, circulat- 
ing waters and deposited as the oxide near the bottom zone of 
surface weathering. 
The Oriskany iron ores of Virginia have been regarded by 
Watson-'^ and Holden^® as having been derived from the iron 
bearing minerals in the Devonian shales overlying the Oriskany. 
These minerals were dissolved by carbonated waters during 
weathering and redeposited in lower levels. Watson suggested 
that the manganese deposits in these rocks might have had a 
similar origin. Accordingly, he collected a sample of unaltered 
shales which overlie the manganese of Bland County to deter- 
mine the absence or presence of manganese. Although the 
analysis showed 4.54 per cent, iron oxides, no manganese was 
found. 
The absence of manganese in the particular sample of shale 
collected has been thought to have made this theory untenable. 
However, despite this fact, we are inclined to the opinion that 
at least some of the manganese was originally disseminated in 
the overlying shales as well as in the sandstones of the Giles 
formation. The reasons for this view may be stated as follows : 
first, some of the deposits occur well up in the Giles sandstone, 
so that derivation, in entirety, from the same formation is rather 
unlikely, and, second, if the iron deposits which are so wide 
spread in the lower measures are derived from the Devonian 
shales, whose analysis shows but 4.54 per cent, iron oxides, might 
not the manganese, whose quantity is negligible in comparison 
with that of the iron, have, at least in part, the same source, and 
yet not be detected in the analysis, especially, of a single sample ? 
Watson, T. L., “Mineral Resources of Virginia,” pp. 408-410, 1907. 
Holden, R. J., U. S. Geol. Survey Bull. 427, pp. 67-68, 1910. 
