Concentration by Wcatlicring. — Kiniha//. 155 
reduction into a more arenaceous or argillaceous and a more highly 
dolomitic rock, and at the same time one which is more irregular in its 
bedding lines. In the true dolomites the shale and sand constituents 
have been evidently of small amount. But in strata where these two 
constituents are prominent, the process would doubtless result in a dis- 
tortion of the sedimentary banding similar to that of the figure. This 
may become, as in this case, the most noticeable distinguishing feature 
of the rock. 
RESIDUAL CONCENTRATION BY WEATHERING 
AS A MODE OF GENESIS OF IRON ORES. 
By James P. Kimball, New York. 
In descriptions of important secondary deposits of sub- 
specular iron ores on the south coast of Cuba in the }'ear 1884,* 
mention was made of other numerous interesting but com- 
mercially unimportant, ferriferous products different in type 
and likewise secondary. These were characterized as con- 
centrations of ferric and magnetic oxides upon outlying sur- 
faces of dioritic dykes, and also developed to some extent 
within a great mantle or overflow of diorite. Involved within 
the same overflow are enormous isolated masses of elevated 
and disrupted coralline rock, some of which in stated circum- 
stances have completely given way to replacement by hematite 
and martite. Further illustrations of both types of deposits 
have also been given by the present writer in a recent number 
of The American GEOLOGiSTf with reference to associated oc- 
currences on islands of British Columbia. Incidental mention 
was made in the same place to similar occurrences in cul- 
minating regions of the Cascade range in Washington. 
Numberless dykes in the foothills of the Sierra Mitstra in 
Cuba, alike in age and original character, have undergone no 
such superficial alteration as above referred to, or, at least, 
preserve no evidence of the kind. That such superficial con- 
centrations of oxides of iron are not due to original raagmatic 
differentiation, on the Soret principle, is clear from the fact 
that eroded tops of intrusive masses and dykes are apt to pre- 
*Am. Jour. Sci., XXVIII, 416; Trans. Am. Inst. Min. Eng., 
XIII, 613. 
tVid. Vol. XX, July, 1897- 
