1877. | 253 
{Prime. 
stone is of a dull, earthy appearance, entirely free from any crystalline tex- 
ture and of a dark grey color. 
Before closing our discussion of the limestone it is necessary to speak of 
the large and numerous deposits of brown hematite iron-ore which occur 
in it, and which form the main support of the extensive iron furnaces of 
‘the Lehigh and Schuylkiil Valleys. ‘ 
The brown hematite iron-ore occurs almost exclusively in two irregular 
lines of deposition; the one along the northern flank of the South 
Mountain Range, the other at or near the junction of the No. II lime- 
stone with the No. III slates. A few other localities occur, at which 
the ore is found, but these are insignificant in number compared to the two 
lines mentioned. Along both these lines the ore is always found either in 
hydromica slate, or resting on limestone very greatly impregnated with da- 
mourite ; the same is true elsewhere whenever the brown hematite is 
found in loco originali. Some deposits are however found which have evi- 
dently been pockets or cavities in the limestone into which the masses of 
limonite have been forced together with gravel and clay during the Drift 
Period. Leaving these out of consideration as of minor importance, let us 
consider briefly those iron-ore deposits which occur in place. It is at once 
evident that like the rocks with which they are associated they are of sec- 
ondary origin, and have been derived from still older formations. The 
occurrence of the brown hematites with silica, alumina, lime, magnesia, 
and the alkalies, more especially potash, points to their having been de- 
rived from Archean rocks containing orthoclase and either hornblende 
or pyroxene. From the decomposition of these three minerals we are able 
to derive all the oxides above mentioned including the iron which was 
without any doubt derived in great part from the decomposition of ferrous 
siliciate present in the hornblende and pyroxene, while a portion of the 
iron may have been derived from iron pyrites, although this supposition is 
entirely unnecessary. It isextremely improbable that the brown-hematite 
was derived from the per-oxidation and hydration of magnetic iron ore, 
when we recall the great resistance which the latter offers to chemicai 
change of any kind when exposed to the action of air and water, and its 
unaltered condition and fresh, bright appearance in rivers and on the sea- 
shore. But the question as to how the brown hematite got into its present 
condition and whether it was deposited cotemporaneously with the rocks 
containing it, or subsequently to these, is still an enigma and various 
theories have been offered in explanation. For a resumé of some of these 
hypotheses reference may be made to a recent article by Prof. J. D. Dana, 
in Vol. XIV, III series Am. Jour. Sci. and Arts, p. 136. The almost en- 
tire freedom of the hydromica slate, when fresh, in Lehigh and Northamp- 
ton counties from ferruginous minerals will prevent our having recourse 
to pyrite, pyrrhotite, chlorite, garnet, mica and staurolite, which Prof. 
Dana says occurs in the hydromica region of Connecticut. Hence we must 
have recourse to other sources. It seems most doubtful that the mineral, 
from which the brown hematites were derived, was deposited cotemporane- 
ously with the hydromica slates in the district under discussion, since we 
