Economic Geological Deposits. — Crosby. 268 
but this type may fairly be retained to preserve the symmetry of tin- 
classification. It should be noted that the essential idea of an economic 
concretionary deposit is that, although the segregating mineral is an 
original constituent of the rock, the segregation is required to make it 
available (concrete) and useful. If the rock, as a whole, is useful, as 
in the case of oolitic limestone and iron ore. ii must be differently clas- 
sified. Clay ironstone is separated from black-band ore in the classifi- 
cation simply because segregation is essential to make the iron carbon- 
ate useful, to change it from a rock to an ore: and the ore only and not 
I he entire rock is used. 
(c) Amygdaloidal deposits, filling the pores (steam-holes) of igneous 
(volcanic) rocks. This familiar type requires no detailed explanation. 
The most important example is, of course, the cupriferous amygdaloid 
of Keweenaw Point: to which may be added the cupriferous felsite of 
Santa. Rita. N. M., although the supposed amygdules may be only 
pseudo-amygdules or actual replacements, filling secondary cavities, 
which would refer the deposit to the metasomatic impregnations. 
(d) Interstitial deposits, filling the pores (interstices) of sedimentary 
rocks. The designation of this type, althoughnot, perhaps, wholly un- 
objectionable, is the most satisfactory thai has occurred to the writer. 
[t is intended to include all those cases where sedimentary rocks have 
been filled but Hot replaced by economic amounts of useful minerals. It 
is very probable, however, that in many cases, as in the cupriferous con- 
glomerate of Keweenaw Point, the interstitial filling is accompanied by 
true metasomatic replace men l. As additional examples of this type, we 
have the cupriferous sandstones of the Copper Basin, etc., Arizona, 
and of various localities in Texas, New Mexico, Utah, etc.; I he silver- 
lead, apparently nou-metasomatic, deposit in quartziteof Coeur d'Alene, 
Idaho: the Potsdam sandstone of the Black Hills impregnated with au- 
riferous pyrite, which may be oxidized: and the argentiferous Triassic 
sandstone of the celebrated Silver Reef mine, Utah. 
The non-concretionary metasomatic impregnations may be 
conveniently subdivided in accordance with the nature of the 
impregnated rock, as follows: 
(e) Metasomatic deposits in calcareous rocks. The Clinton iron ores ap- 
pear t"o be, in part, limestone replaced by ferric oxide. The deposits of 
galenite and nickeliferous pyrite of Bonne Terre, Mine la Motte, and 
Doe Run, Mo., are local enrichments (impregnations) of limestone; and 
the silver ores of Glendale, Montana, and Wood River, Idaho, clearlj 
belong under this type. According to Kemp's recenl and able summary 
of our knowledge of the extensive and unique deposits of oxidized zinc 
ores al Franklin Furnace* and Ogdensburgh, X. .1.. they are most prob- 
ably interbedded impregnations. 
(f) Metasomatic deposits in frag mental rocks. Some of the native cop. 
per of Keweenaw Point belongs here: and the copper ores near the i rap 
-Trans. X. Y. Acad. Sci.. xm. pp. 76-98. 
