ORIGIN AND CLASSIFICATION OF ORE DEPOSITS. 165 
the Silurian deposits. These present a close resemblance to those now living. 
Prof. Huxley tells us* that only one order of the corals has become extinct. 
Any one looking over the beautiful volume of Zoophites, by Prof. J. D. Dana, 
compiled from his researches while connected with the Wilkes exploring expedi- 
tion in the Pacific, will be struck even more by the diversity of conformation than 
by the beauty of colors in this branch of animated nature. He describes over 
five hundred species (we quote from memory) and saw as many more which he 
had not time to classify. Agassiz in 1850 estimated that there were ten thousand 
living species of Radiates. 
THE ORIGIN AND CLASSIFICATION OF ORE DEPOSITS. + 
BY PROF. J. S. NEWBERRY. 
The mineral matters which have proved useful to man form three categories : 
first, the earthy, as gypsum, clay, marble; second, carbonaceous, as coal, 
lignite, petroleum ; third, metallic, as iron, gold, silver. 
The metals occur rarely native, oftener as ores, that is, combined with sul- 
phur, silica, carbonic acid, etc. ‘These form a series of deposits, of which the 
physical and chemical characters and history differ widely. They may be grouped 
into three classes, as follows: 
1. Superficial Deposits. 
2. Stratified Deposits. 
3.  Unstratiped Deposits. 
SUPERFICIAL DEPOSITS. 
These include the accumulations of gold, stream-tin, platinum, gems, etc., 
which are obtained from the surface material, gravel, sand and clay, derived 
from the mechanical decomposition of rock masses through which metals or ores 
were sparsely distributed. Thus, gold usually occurs in small quantity in the 
quartz-veins of metamorphic rocks. By the erosion of these rocks, having been 
freed from its matrix, and that more or less perfectly removed, this gold is con- 
centrated by a natural washing process similar to that employed by man, but on 
a grander scale. Inthe same manner, the oxide of tin, which is hard, heavy 
and very resistant to chemical agents, is distributed sparsely through granitic 
rocks or vein-stones ; and where these have been eroded, the cassiterite remains 
in the alluvial deposits of streams, where it can be cheaply and easily collected. 
Superficial deposits have probably furnished nine-tenths of all the gold that 
has been obtained by man, the greater part of the tin, all the platinum and its 
associated metals (iridium, osmium, etc.), and all the gems except the emerald, 
which in South America is obtained by mining. ‘Thus, it will be seen that the 
surface deposits are scarcely less important, economically, than the others. The 
*Lay Sermons, etc., X p. 216. 
{From the School of Mines Quarterly for March, 1880. 
