Genesis of [ron-Ores. — KhhhalL 369 
direct replacement; and to attach what is conceived to be due 
significance and importance to such circumstances. 
Chemical replacement of limestone by ferrous carbonate, it is 
hardh' necessary to remark in review, proceeds from surfaces of 
cleavage and stratification, wherever penetrated by infiltration of 
atmospheric waters from neighboring permeable ferruginous rocks 
like schists and shales: or. again, from surfaces and interstices of 
rocks, wherever calcic carbonate is produced by decomposition of 
silicates. Or. still again, solutions of lime and iron salts from 
one or the other source, give scope to similar and successive re- 
actions. Hence transmutations within compass of l)oth limestone 
and adjacent schists. Mixed detritus from ])oth kinds of strata, 
and overspreading consideraVde surfaces lielow the level of de- 
velopment or outci'ops of such strata, also give place to like re- 
actions. Hence numerous shallow deposits of limonite at the 
base and on slopes of Appalachian ridges. Much of the ferric 
hydrate which appears due to tardy precipitation at considerable 
distance from sources of iron-salts is doubtless from indii'ect re- 
placement of limestone schutt. 
TV//. Rcphici'niitit of Canihilaii T/iuirsfnius. Luhi- Superior 
rrtjioi). iind Mhnoiirl. 
The notable occurrences of crystalline iron-ores at Iron Mount- 
ain, Shepherd Mountain, and Pilot Knot) in Missouri, within 
developments of stratiform porphyritic petrosilex (hiiUaflinta) 
was ascribed by Pumpellj', in the year 1 S72. to replacement of 
limestone.* Development of the petrosilex was also supposed 
to have resulted from chemical processes of replacement. How- 
ever this may be. it is not dirticult to imagine development of 
petrosilex from a siliceous limestone, or from a limestone inter- 
calated Avith siliceous matter, concurrentl}' with replacement of 
the calcareous material by iron-ores. Developments of this kind 
may l)e regarded as akin to the jasi)ery foliations of petrosilex 
along Avith crystalline ores in the Lake Superior region, the genesis 
of which by replacement of calcic carboiuitc has recently been 
traced by other geologists. 
The relations of specular or iiematite schists in the Lake Su- 
perior regicni to sidcrite. and the phenomena of its original de- 
vehjpment and distribution have recently been treated, first, by 
*Geol. Surv. of Missouri. 1878: Iron (Jres, 2f5. 
