Gen^iils of Iroa-On^s. — KimhaU. 361 
VI. Llmonitrs (and crys^taJl inc iron-ores) of the Loirer Silnn'mi 
limestones. 
That epigenic relations of some sort subsistbetween the limou- 
ites and the Lower Silurian limestones with which they are asso- 
ciated throughout the Appalachian valleys, was distinctly pointed 
■out by W. B. Rogers over half a century' ago. Their relation 
with ferruginous shales and sandstones alternating with the lime- 
stones is almost equal!}' intimate. Not only was the epigenic or 
secondary origin of these limonites recognized b}' Rogers, but 
their mode of occurrence, that is — not as consecutive members of 
formations between which they outcrop and often appear to be 
imbedded, but as irregular accumulations comparatively shallow or 
altogether superficial. Sometimes lining, so as to occup\'. 
-enlarged fissures of stratification for a limited extent, they 
are still liable to l)e mistaken for regular or ])edded depos- 
its. Others, occupying in like manner enlarged fissures of 
cleavage or jointing in limestone, are sometimes assumed to l)e 
•of the nature of segregations or mineral lodes. But their 
most common occurrence is in what is often described as super- 
ficial basins or other depressions on imperfectly drained sur- 
faces and slopes upon a limestone floor, in proximity to lime 
stone, or within compass of its extended lines. This superficial 
association of limonite with the Silurian limestones themselves is 
sometimes considered to be simply owing to the cavernous condi- 
tion of limestone, and to its liability to une(}ual erosion; whence 
the occurrence of local depressions, or sink-holes produced by 
subsidence, which .have eventually become repositories of iron- 
ores from purely adventitious precipitation of ferric hydrate 
from passing waters. Pre-existing caves and crevices are like- 
wise assumed to be so filled out with this deposit, or even by 
mechanical accumulations or ■in-washes' of ferriferous material.* 
To the general, or even a wide, application of such a view to the 
occurrence of important deposits of iron-ores serious objections 
are opposed. First, caves and cavities of this kind have been 
produced by solvent action of circulating water. Second, what- 
ever insoluble products, including ferric hydrate, are separated 
from moving waters remain in suspension and finally escape. 
Third, possession of superficial depressions by detritus, calc- 
schutt, etc., prevents them from becoming open receptacles of 
*Newberry; School of Mines Quarterly, Nov. 1880, Reprint p. 10. 
