354 21ii Aiin'rif'tin Orohx/lsf. Dcc.mhIjit, wn 
not less than !lt)l!.tMiS tons <»f iion-oii'. the procluct of a sinjik' 
operation, that, namely, of the Jnra<iua Iron Company. This 
product was over one-(|narter of the total imports of iron-ores in 
the United States for the same i)eri()d. 
Till' sanu' examples of molecular rejjlacenient on a »>r;nid or 
petrographic scale will serve to illustrate deix'udence of this mode 
of epigenesis on circumstances of environment. These are seen 
to ha\e lieen }tarticularly favoialile. as was also the extreme 
l)orosity or permeability of the orijrinal coralline material. Suffice 
it to mention (1) enveloi)ment of nnisses of this material with 
hasic erui)tive diorite. rich in iron silicates: and i'l) climatic con- 
ditions specially conducive to secular weatherinu' of superficial 
rocks. 
Some of the iron-ore bodies exhibit concentric structure, a re- 
sult of progressive weathering — the same as exfoliation on a 
smaller scale — easily mistaken for quaquaversal dips. Numerous 
shrinkage fractures due to contraction incidental to alteration, as 
well as larger manifestations of mechanical fracture, are filled 
out with chloritic material from decay <)f the tlioritic magma and 
detritus. Thus masses of iron-ore are reticulated with liands of 
chlorite. 
Incidental to secular decay of the mantle of dioriti' enveloping 
)nasses of limestone, chemical interchange or double decomposi- 
tion was effected l)etweeu fixed calcic carbonate and dissolved iron- 
salts. Ferrous carbonate, directly passing into ferric hydrate, 
was left behind, and lime salts were dissipated. The partial de- 
hydration of ferric hydrate into ferric oxide (turgite), the product 
of further alteration, may. as shown by Davies and Rodman, 
proceed from only slight elevation of temperature.* There is no 
evidence, however, that these ore-bodies have been deeply buried. 
Indirect replacement in this instance lias therefore ln-en under 
exposure to an oxidizing atmosphere. 
The metasomatic development of the iron-ores at the l)ase of 
the Sierra Maestra dates from :in era no more remote than the 
Cenozoic. when this coast range was added to the island of ('nl)a. 
IIT. lirpltuftiK )it of Ctirboni/eroxs ami Slhiruin Iyiiiusf<j)ies, 
Colorado. 
Mr. S. F. Emmons has shown extensive replacement of Jiower 
Carboniferous and Silurian limestones to have resulted in occur- 
*Jom\ Chem. Sor. ir, iv, 09. 
