Gene><ls of Troii-Ofi.\. — KinJxi^l. 367 
hj'drate — as determiiuMl by atiiiospherie environment. This re- 
action may l)e assumed to 1»e common V)oth to limestone and ad- 
jacent transition scliists. and even less closely related siliceous 
schists. 
Iron sulphides, sometimes visible in sound Lower Silurian 
limestones, seem to be developed in about the same minute pro- 
portion that ferrous carbonate is often revealed l)y analysis. 
Limonite develojjed 1)y epigenesis on horizons of these limestones 
iind of adjacent schists commonly atfords traces of sulphur, as 
pointed out by Mr. Prime. 3Iuch laroer proportions of iron sul- 
. })hide are notaldy developed in the schists. 
It may therefore be concluded that both immediate and ex- 
traneous sources have contriljuted to fixation of ferrous carbon- 
ate as now found in parts of thyse limestones and associated 
schists: and that iron was received 1)V these sediments in a car- 
bonated atmosphere before or durins: their consolidation as well 
as afterwards. 
This conclusion seems to be in line with Dana's* numerous ol)- 
servations in western New England and eastern New York, as well 
as with those of the Second (ieolouical Survey of Pennsylvania, 
and to fall in with certain general deductions of Mr. Pi'imet and 
of Dr. Julien.i 
Desides limestones of the Sihiro-Cambrian series, other lime- 
stones of the Silurian and Devonian periods give identification to 
occurrences of epigenic limonitic iron-ores. These are irregular 
shaped masses often described as lenticulai' (but in an opposite 
sense of that term as applied in this memoir) Ix'tween limestone 
and adjacent ferruginous strata. Thus occupN'ing bordering 
divisions of strata, they not infriMpiently assume to partial view 
semblance of interstratified deposits with which they have some- 
times been confounded. 
■ Such relations, liowevei'. })oint nioi'e or less distinctly to re- 
placement of calcareous matei'ial through essentially superficial 
agencies, and to circumstances of attitude aiul environment 
favoral)le to epigenesis of linionile. These circumstances are 
ordinarily less obscure and complex than th(;se which govern the 
♦Besides the several papers by Prof. Dana alreadj- cited, touching the 
present subject, inav especiallv be mentioned erie on Berkshire Geolog)', 
(Berkshire Hist, and ^c. Soc, Pittsfield, 18^f(i.) 
tAin. Journal of Sc. i,.\, 187."), 432. 
*An. New York Acad. Sc. Ill, ISMii, :5<j:j, .-JO!). 
