488 F. Prime, Jr.—Limonite Deposits of the Great Valley. 
The damourite-slates, being the products of the decomposi- 
tion of primary rocks containing orthoclase and probably oli- 
goclase, or albite, were deposited in the Silurian sea during 
the Calciferous epoch and near its commencement. At Allen- 
town and Bethlehem, Penn., the slates are but 50 to 100 feet 
above the Potsdam sandstone, with a blue massive limestone, 
generally crystalline, between the two. Limestone was again 
deposited above the slate. I am not yet prepared to say 
whether more than one bed of damourite-slate was formed, and 
reserve an opinion on this point until I have made further 
explorations. 
he brown hematites were probably formed by the oxidation 
of iron pyrites, but the former are not in the same place that 
the latter were. My reasons for this assertion are twofold. In 
the first place, the animal carbon of the organisms, whose shells 
formed the enormous quantity of dolomite or limestone, exists 
in such quantities throughout the limestone as to color the 
rock, the zinc -blende (of Friedensville) and the carbonate of iron 
a bluish-grey. This same carbon would readily reduce the sul- 
phate of iron carried into the ocean to iron pyrites. As a proof 
of this reduction I need only mention that iron pyrites have 
been found in the mud* of a pond. Secondly, the great major- 
ity of the brown hematites which I have had analysed, and they 
are many, contain a trace of sulphur, usually not more than a few 
hundredths of a per cent. have also found minute (almost 
microscopic) crystals of iron pyrites in much of the limestone, 
where it is opened in quarries; and I have also seen in much 0 
this same limestone, where weathered, minute cavities, which I 
have ascribed (perhaps erroneously), from their general appear- 
ance, to the decomposition of the pyrites. 
t is at present impossible to say whether the pyrites from 
which the brown hematite ores were and are forming were thus 
minutely disseminated through the limestone, or whether there 
was a bed of the limestone—now decomposed,—especially rich 
in the pyrites. 
In either case it would seem that the pyrites above water- 
level—and we must bear in mind the great erosion which the 
surface of the country has undergone since it was formed— 
would oxidize from the action of the water and the air carried 
in by it, forming protosulphate of iron. This being readily 
soluble in water, was carried down through the limestone, form- 
ing sulphate of lime (gypsum) and carbonate of iron wherever 
the solution came in contact with the fresh limestone. This 
reaction was, however, probably slight, owing to the rapid 
descent of the solution in seeking the water-level. It experi- 
enced no difficulty in its descent until it came in contact with 
* Gmelin—Kraut’s Handbuch der Chemie, 6th ed., vol. iii, p. 333. 
