Genenh of Iron-Ores. — Kuiihall. 365 
in intercalated or adjacent schists. Xunierous analyses by the 
Second (leological Surve}' of Penns3dvania indicate the presence 
of sulphur in almost all of the ores of the same belt in that 
State. The dissemination of visible pj'rite throughout unde- 
composed parts of both limestone and schists is also noteworthy. 
This seems to be in about the same minute proportion that fer- 
rous carbonate is often similarly repi'esented. The epigenesis of 
the latter is probal)ly from pyrite. But hitherto the most remark- 
able evidence of pyritous material in the development of limon- 
ites associated with the same limestcjne series has been found in 
Alabama. In that region concentrically weathered or foliated 
masses of limonite isolated on the hill-tops b}' erosion of enclos- 
ing clay, the residuum of decomposed schists, sometimes retain 
nuclei or cores of undecomposed and highly pyritous rock. Cen- 
tral portions of other masses of limonite consist of siderite. 
Hence the not uncommon occurrence in limonite workings in 
limestone of ribs, masses and nuclei of siderite, ferro-calcite or 
limestone, from incomplete replacement, or incomplete sulisequeut 
alteration. Hence also similar occurrences of pyritous residn urns 
in limonite developments in adjacent or transition schists. 
In given instances, l)e the mode of replacement of limestone 
by ferrous carlionate what it may. whether from styptic or chah'- 
V)ic solutions, or whither originally or eventually introduced into 
Siluro-Cambrian limestones, in the lower member of which series 
(Calciferous) limonites have l>een so extensivel}' lirought to light 
in Appalachian valleys, there seems much reason to conclude, 
as sometimes held, that in the course of chemical erosion of the 
limestone and of decay of related schists, very considerable 
accumulations of residual limonite have resulted from alteration 
of diffused ferrous carbonate or pyrite. not exct'teding two and oiic- 
half per cent., to stable ferric hydrate. Parts of the same 
series exposed to weathering or erosive action, and commonlv 
below the full thickness, seem, in certain instances at least, to 
bear some proportion to the thickness of related accunuilations 
of limonite. That is. the vertical range of the residual limonite 
seems to be in proportion to the thickness of limestone eroded, or 
ratlu'r to the measure of its shrinkage. Yet along with the es- 
cape of dissolved lime and m.ignesia salts in ni.Msure of chemi- 
cal erosion, there is also dissipation of iron salts. 
According to Lesley, limonite or pipe-ore. so-callel. has a 
