(rrni'slK of Iriin-( )r<:>^. — KiiiJmJI. 363 
is, and even concurreiil with sedimentation <jf the material of 
these massive limestone formations. Any chancre of sedimenta- 
tion, snch as contemplated in tliis theor}', even if chemically 
snpposaljle. wonld have resnlted in lenticular configuration of 
siderite, or of its derivatives, a feature as al)ove jjointed out, 
probably possessed by siderite only in the case of complete or 
pseudomorphous replacements of lenses of limestone. 
I have, in a previous i)a})('r. descrilnMl lenticular occurrences of 
siderite at IJurden, Columltia county, N. Y., on the horizon of a 
thin non-persistent and jjrobably estuarine Lower Silurian lime- 
stone.* Whatever theory l)e entertained of their epigenesis. this 
must l)e perceived to Ije closely connected witii overlying clastic 
ferruginous accumulations, the whole series occupying in-shore 
depressions in an undulating bottom overspread with calcareous 
material. In the paper referred to. these basin-like deposits, 
which in point of identification with Silurian nuMnbers. are. I be- 
lieve, so far as known without parallel in this (-(junti'v. were sub- 
stantially explained as i)rol)ably instances of phijxicitl replace- 
ment of limestone l)y ferriferous accumulations, graduating up- 
wards into ferro-calcareous grits. The transgressive character of 
"these grits appears from the fact that their development is limited 
to the basins themselves. Their thickness in any given vertical 
section is [)roportioual to the thickness of the ore. The grits as 
well as the siderite are distinctly lenticular. The siderite was 
considered as a metamorphic product from reduction of ferric 
hydrate in reaction with commingled organic matter. 
Having since come to the conclusion, through a critical exami- 
nation, in another place followed out, of the several tiieories of 
epigenesis of siderite to be found in treatises and text-liooks, that 
all are at variance with chemical observations, or opposed by objec- 
tions on ph3'siographic grounds, T am now forced to l»elieve that 
these interesting occurrences reciuire a different explanation, 'i'hey 
seem, indeed, to present no exception to the explanation of epi- 
genesis of siderite by chemical replacement of alkaline mono- 
carbonates, but on the contraiy to aUord striking examples 
of complete or pseudomorphous r('|)lacement of limestone 
beds. 
Indirect replacement of linu'stone (U) l)y limonite at the Hurst 
ore-bank, AVythe Co.. ^'a.. has been graphically described by 
*Am. Jour, of Sc. xl, 1890, 150. 
