"25i The American Geologist. April, 1894 
sidering the relations of the deposits to each other or their 
genera] relations to geological history, gypsum stands natur- 
ally much nearer to slate than to tin ore; and genetic, struc- 
tural, and economic considerations all demand that superficial 
and deep-seated deposits should not be commingled. 
In the opinion of the writer, we have good reason to believe 
that fusion and solution arc not only continuous and overlap- 
ping processes, but that giant granite or pegmatite may In- 
regarded as marking the halfway point, the product of their 
perfect cooperation. Pegmatite combines the characters of 
plutonic and vein rocks, but does not belong distinctly to 
cither class, demanding separate recognition and classifica- 
tion. 
Explanation ok the Classification. 
The classification embraces, as the table shows, three pri- 
mary and coordinate divisions, as follows: — 
A. Deposits <it' igneous origin (igneous rocks). Heal is the 
chid' agent. 
1!. Deposits of aqueo-igneous origin (giant granite or peg- 
matite). Heat and water cooperate. 
('. Deposits of aqueous origin (sedimentary and vein rocks). 
Water is t lie chief agent. 
These main divisions of the classification may now be con- 
sidered separately, and their subdivisions noted, with illus- 
trative examples of each type, so far as such have been clearly 
recognized. Many of the examples, it should be added, have 
been taken directly from Kemp's -'Ore Deposits of the United 
States-/' and his interpretation of the facts have, in most 
cases, been accepted without further investigation. 
A. Deposits <;/' igneous origin. — Following the lines of the 
lithological classification, we may divide the igneous rocks 
into the intrusive or plutonic rocks and the extrusive or vol- 
canic rocks; although this distinction is obviously not of par- 
amount importance on purely economic grounds. These two 
great classes of igneous rocks admit, for economic purposes, 
parallel subdivisions, as follows: 
i. Igneous deposits of subterranean origin (plutonic rocks). 
Among the plutonic rocks we may recognize: 
(a) Normal plutonic rocks, altered and unaltered. These are useful es- 
pecially as affording materials for construction (building and ornamen- 
tal stones, etc.): hut fchev have in the aggregate, i>\ virtue of their 
