3H(} The Anioi'lcan Geologist. D.-comber, isgi 
liiiu'stoiu' of Xi'wl)(.'rrv. iiad tlir ovi'rlyinjz dori" liim'stoiic of 
the I locking ^'lllloy — both members of the Mercer grou[) of 
strata. 
Many well known horizons of siderite in J'cnnsylvunia and 
liordering States have been described by Stevenson. The Mount 
Savage ore-group of the Conglomerate series is worthy of men- 
ti<»n.* One plate or more of siderite occurs in the Mauch Chunk 
it'd shale. In many places local lenticular stratiform plates of 
siderite extend for a few hundreds to a few thousands of feet 
with a thickness up to four feet. In the Vespertine of (ireen- 
brierCo. . of A'irginia, an occurrence of this kind has been de- 
scribed by Rogers, and more recently Ity Mr. W. X. Page.t 
Comparatively thin, lenticular or stratiform developments of this 
description may be assumed to be complete replacements of non- 
persistent limestones of estuarine origin. 
Incomplete replacements of Carboniferous and Sul)-carl)on- 
iferous limestones of the Appalachians are, on the other hand, 
devoid of terminal edges, or other well defined demarcations such 
as appertain to lenticular beds conformable to the c(^)nfigurati<>n 
of hydrographic basins. They are thus perceived to be wanting 
in characteristics of sedimentary or metamorphic deposits. 
This obviously would not be the case, if these developments were 
primarily' due to mechanical or chemical deposition in the natural 
order of succession of beds h\ which they are enclosed. That 
they do not occupy pre-existing cavities produced 1)}- mechanical 
or chemical erosion is likewise certain from the fact that they 
sometimes nu'rge into limestone. That they are not crystalline, 
or characterized b}' other pluniomena of segregation, is conclu- 
sive negative evidence of some other origin. Thus, there seems 
to V)e no alterinitive but to consider these developments due to 
molecular replacement of limestone, of which mode of origin 
indeed there is no lack of positive and direct proof. Hence, in- 
<luctively. again the conclusion that comi)lete or strictly i)seudo- 
morphous replacement of limestone has been wrought in the case 
of stratiform developments of siderite and its derivatives, the 
iron-ores still retaining perhaps physical features of the original 
limestone in common with stratified deposits. 
♦Second CJeol. Survey of Peim. KK. 
fTraus. Am. Inst. Min. Eng. 1888, E.xtract, p. 4. 
