368 The American, Geologist. T).ceini).>r, isot 
<li.slribution of accumulations of iron-oro eonuccte(l w ith the lower 
series of limestones, mainly on account of the distinct configura- 
tion nnd uiHlistiiiltcd relations of the nccuniulations themselves. 
I //. Jirplainnciif of (^rijstnll i ii< iraii-oris. 
According to Ijcsley and d'luvillers the famous magnetite de- 
j)osits at Cornwall. Pennsylvania, like the developments of brown 
hematite on the same horizon throughout the (ireat Yalley. were 
originally a formation of Lower Silurian magnesian limestone beds. 
••Tt nuiy safely l»e said," as remarked by these ol)servers. 
'•that the Cornwall ore innss has experienced three stages of 
development; being originally a formation of lime-shales; then 
becoming a great brown hematite formation ; and finally a mag- 
netic ore-formation.""* 
The advanced metaniorphism of the original ore-replacement 
from limonite, or perhai)s from siderite, to magnetite, is a 
phenomenon which there is much reason to l)elieve. is connected 
with lo(;al doleritic intrusions along the edge of the Pennsylvania 
Mesozoic, as also in the case of other occurrences of magnetite 
on the same horizon at Boyei-stown. Dillsburg :nid elsewiiere in the 
same State. 
Replacements of metanioi'phic limestone liy other types (»f 
crystalline iron-ores linve recently been ol>served by me in St. 
Lawrence County. X. V. Here the Calciferous limestone luis 
become crystalline, and its ore-replacements have l)econie meta- 
morphosed into red hematites or anhydrous ores of the specular 
type. At Pierrepont, highly crystalline specular ore, of s|)len- 
dent lustre, occurs in white metamorphic limestont' of Calcif- 
erous iige. 
Taken in connection with my previous menioii' on the present 
sul)ject, the foregoing facts, here very l)rierty considered, tend to 
prove that I'eplacenient of Pahvozoic limestones, and other cal- 
careous material, inchuling c'llcic carbonate fi-oni decay of 
siliceous schists, h;is been wrought indifferently from botii in- 
filtrating chalybic and styptic waters, through surrender in eneii 
case of a weak l):ise like ferrous oxide for :i strong l);ise iiki' lime. 
I have also sought to indicate some of the circumst:inces of :it- 
mospheric environment, topogniphy or vicissitudes governing 
preservation, or — on the other hand — alteration of the product of 
*An. Rep. of the (teol. Surv. of Penn., for 1885, 188(1, p. r)8n. 
