•"••'•^ lln' Auirrini II (i/ohx/isf. l)i(.-iiib(_T, 18!»l 
uiiiuixcd rli(>mic:il pivc'ipitjitcs. lUsplncoini'iit of detritus must 
llicrofoic pivi'oiU' st'puniti' deposition of homogeneous material, 
except Ity molecular replacement of limestone surfaces, calcar- 
eous detritus and calc-schutt. IMiysical displacement happens, if 
at all. 1)y means of torrents, from which again no chemical' 
deposition ImUcs place except possibly l>y ri'placement, or at least 
only for a time. Important deposits of limonite free from de- 
tritid admixture, even of clay, ordinurily. at least preclude an 
exphiiiation of their niecluinical iiccumulation in pre-existing 
ciivities. 
Yet it is true that iimoiiii- the great varii'ty of circumstances of 
environment ;ind topography in which development of limonite has 
taken place on horizons of Lower Silurian limestones, instances 
there are where c;i\-es. cavities and enlarged crevices, once chan- 
nels of rapid tninsniission of waters, have evi'utually been oV»- 
structed liy fulls of rock or otherwise, and, so cut oft* from 
drainage. li;i\c liiuilly become ([uiescent ri'servoirs of mineral 
waters. In tiiis w;iy vaciint spaces in calcareous beds ma}' in 
some instances have l»ecome repositories of uninterruptetl deposi- 
tion of ferric hydrate either by precii)itation. or by indirect re- 
placement of limestone surfaces or calcareous contents. Limon- 
ite :is sometimes occuriing. and bearing evidence of having 
probably l»een formed in this way. li;is j)roved not uncommonly 
m(n'e enduring than tli*' limestone itself, the destrnctive erosion 
of which it luis sur\i\H'd in the form of outlying masses. 
Onbnary nccunudations of limonite thrown down in caverns, 
sink-holes :iiid other depressions along with detritus, within the 
limestone ore-ltelt of Penn.sylvaiiia. liaxc liecn well characterized 
V)V yiv. d' 1 n\illiers.*'' Some of the obstacles in the way of a be- 
lief that i»i('-existiiig ca\ities often atford lodgement to import- 
ant (»re-dep(»sits liaxc been aptly stated by Kmmons.f 
['naltered sideiite in irregular shaped masses, sometimes oc- 
curs in the midst of limonite in ore-banks upon horizons of the 
Lower Silurian limestones as in Columbia and Dutchess counties, 
X. Y. . at liichniond and West Stockbridge, >Iass., and at a few 
points ill I'eiiiisylvania and N'irginia. These occurrences af- 
ford whatever grounds l»e found for a theory of original 
deposition of concentrated ferrous carbonate, intermittent, that 
*Second Geol. Surv. of Penn. An. Rep. 1880, iv, p. 418, 
tTrans. Am. Inst. Min. Enir., 1880, Ex. p. 5. 
