14 The American Geologist. July, is97 
In replacement of pyroxenic and lime-feldspar rocks of tlie 
greenstone family, like diorite, diabase, gabbro, etc., and oth- 
er basic rocks, whether metamorphic or intrusive, residual 
concentration or fixation of stable iron oxides is brought a- 
bout through initial decomposition of ferrous and ferric sili- 
cates, chiefly the former, whence development of soluble al- 
kaline carbonates along with ferrous salts, whence again spon- 
taneous double decomposition, and finally isolation of insol- 
uble residues. This cyclus or process of weathering action is 
obviously regenerative. 
Secondary occurrences of the kind referred to have come 
to be described in general terms as differentiations of iron ox- 
ides not only in concrete ore bodies of irregular or lenticular 
form between divisional surfaces of eruptive masses as well 
us of terranes, but also in graduations of. rock into ferrifer- 
ous aggregations. The same term however as applied to pri- 
mary concentrations of highly basic products in eruptive rocks, 
as may easily be conceived, is by some writers (Brogger, 
Vogt, and others) even made to imply isolation of magnetite 
as a mode of development of certain ore-bodies from cooling 
magmas. An objection of weight to a theory of this kind a- 
rises from the invariable instability ©f basic rocks of all ages, 
and from their profound alteration by hydro-chemical molec- 
ular or atomic rearrangement, involving all constituents. 
In replacements of pyroxenic rocks whether superficial or 
under cover an environment of limestone, even where no bod- 
ily replacement of limestone has occurred, is so common as to 
have a significance beyond mere coincidence, and for obvious 
reasons. Equally significant is the notable presence of lime, 
sometimes as a carbonate, in basic crystalline aggregates, like 
diorite, dolerite, diabase, gabbro, etc., — most subject to alter- 
ation, and, in physical circumstances favorable for preserva- 
tion of such secondary deposits, the particular repositories of 
ferriferous concentrations. 
Both kinds of replacement are alike external and intersti- 
tial. While replacement of limestoae seems to take place in- 
differently from without inward, replacement of siliceous pro- 
ducts seems to be primarily determined inversely to the meas- 
ure of local drainage and in ratio of interstitial space either 
actually presented or potentially developed. The latter we rec- 
