i62 Tlic Amciican Geologist. March, \m>^ 
initial replacement of calcic carbonate through ferrous car- 
bonate — both products of the splitting up of basic silicates, 
the original components of which were not in stochiometric 
proportions. The last inference may be drawn from the 
amorphous condition of the ferriferous beds as well as from 
analyses. The only evidence of any other kind of dififerentia- 
tion is occasional segregation of calcite. 
In the present example the transformation is obvious. 
Equally obvious, of course, it would not be had its progress 
involved the whole formation, or even uniformly a single- 
division. In the bearing of this example on differentiation 
and concentration of ferriferous products from basic and 
somewhat magnetic aggregates, it is uncommonly instructive. 
Similar differentiation from eruptive basic magmas in Cuba 
previously instanced as a product likewise of weathering ac- 
tion, affords a further illustration of a common mode of 
genesis of iron ores from basic material. 
It is natural to infer that superficial and even interstitial 
concentration of stable magnetite incidental to hydro-chemical 
rearrangement and leaching of basic rock, of which this 
mineral is a component, may be largely, if not wholly, res- 
idual. Apart from such a mode of occurrence as distinguished 
by special paragenesis, its presence in concrete form in non- 
magnetic rocks, in numerous cases instanced by the writer in 
previous pages of The American Geolo(;ist. is through 
stochiometrical transformation of ferric hydrate at ordinary 
temperatures. Transformation of this kind by gradation 
proves to be among the more common microscopic manifesta- 
tions of epigenesis of basic crystallines, attended with loss of 
basicity. Diminished basicity seems invariably, so far as I 
have observed, and as I have instanced in numerous descrip- 
tions, to characterize and differentiate regional parts of rocks 
marginal to concentrations of anhydrous iron oxides, and as 
particularly witnessed in development of secondary siliceous, 
or residual products like chlorite, epidote, garnet and jaspers 
from gabbro, diorite, diabase, etc. Superficial, unlike inter- 
stitial, concentrations in basic rocks seldom, if ever, appear of 
economic importance. Far more important products are 
those derived from essentially calcareous paragenesis when 
iron salts, locally generated and entering into passing solu- 
