266 The American Geologist. April,i894 
tions from the Cambrian to the Tertiary; the specular hematite of the 
Marquette and Menominee districts (probablj I; the Vermilion lake hem- 
atite, believed by Profs. N. H. and II. V. Winchell to be a marine chem- 
ical precipitate; the great bed of specular hematite on Pilot Knob, Mo. 
(probably); bauxite and many manganese deposits. 
(b) "Reduction. The chief agenl is organic matter, and under its influ- 
ence the soluble sulphates yield native sulphur and various metallic sul- 
phides. Sulphide of iron is extensively deposited in this way in argilla- 
ceous sediments; and il may be that some of the greal beds and lenses 
of pyrite have had this origin. 
(c) Evaporation. The chief examples are siderite (black-band, etc.), 
calcareous and siliceous tufas, gypsum, rock salt, etc. The tufas would 
he of special interest in a more detailed study as marking the overflow 
portions of subterranean solutions which formed veins and impregnal ions 
on their way to the surface. 
3. Organic deposits (organically formed rocks, etc.). — 
These have been formed by the accumulation of: 
(a i Hydrocarbons, giving rise to coals, bitumens, etc. 
(b) Siliceous organisms, giving rise to tripolite, etc. 
id Calcareous organisms, giving rise to limestones. 
idi PhospJiatic materials, giving rise to guano and phosphate rock. 
4. Metamorphic deposits. — Either of the above groups (1, 
2, 3), under the combined influence of water, heat, pressure, 
and chemical change, may become metamorphic (crystalline). 
affording thus : 
(a i Normal metamorphic rocks, where the rock as a whole may he use- 
ful (some of the crystalline iron ores belong here); and 
(hi Original disseminations in metamorphic rocks, where crystallization 
has developed in the rock minerals (usually accessory, such as garnel 
and pyrite) which may be of economic interest. 
5. Residual deposits. — These are due to superficial chem- 
ical changes (oxidation, hydration, etc.) and commonly to dif- 
ferential chemical erosion. Two principal types may be 
recognized, as follows : 
(a) Oxidized and partially o.r;<li-_i</, and residuary metalliferous deposits 
(including gossan). The chief examples are the residuary ores of iron 
and manganese, including the limonite'deposits of the Great Valley, the 
great deposit of magnetite at Cornwall. Pa., being, perhaps, a metamor- 
phic example of one of these. The Clinton ore (hematite) is due. in 
part, to the differential erosion of ferriferous limestone. 
(l>) Residuary earthy and siliceous deposits, including a large proportion 
of the ordinary superficial detritus or soil and some phosphate deposits, 
kaolin being the most important single example. 
In conclusion, it may be noted that it is not ;i valid argu- 
ment against this or any classification that our ideal types are 
