168 KANSAS CITY REVIEW OF SCIENCE. 
in some cases the ore or metal has been segregated from one or the other of the 
strata at the time of their deposition, and in others it has come from a foreign 
source, and has been deposited in a more or less continuous sheet in cavities 
formed between the surfaces of the adjacent rock-beds. To the second of these 
classes would seem to belong the argentiferous ores of Leadville, Colorado. 
These are deposited along the plane of junction between an underlying limestone 
and overlying porphyry, and undoubtedly accumulated in vacant spaces formed 
by the solution of the limestone. ‘These ore bodies have apparently much in 
common with the pockets and chambers excavated in certain limestone beds, 
and subsequently filled with ore, to be described farther on. ‘The true structure 
of these Leadville ore bodies can, however, only be accurately learned when 
they shall be penetrated below the zone of unchanged sulphurets into which they 
will undoubtedly merge in depth. 
The term Stockwork is applied in the old world to a mass of rock or vein- 
stone penetrated in all directions by small intersecting sheets or veins in such a 
way that the whole mass is mined out. Some examples of this kind of deposit 
may be found in most of our mining districts; but the most important which have 
come under my observation are in the Oquirrh Mountains, in Utah, and at Silver 
Cliff, Colorado. In the first of these localities, beds of quartzite—in the second, 
of porphyry, have been shattered, and the crevices between the fragments have 
been filled with ore deposited from solution. 
The name Fahlband, or rotten layer, originated in the silver mine of Kongs 
berg, in Norway, where there are parallel beds of rock impregnated with the 
sulphides of iron, copper, zinc, etc., which, by their decomposition, have rendered 
these beds so soft as easily to be removed. We occasionally meet with pyritous- 
rock in this country, which decomposes in the same way, but none yet known to 
me has any considerable importance as a metalliferous deposit. 
Impregnations may be defined to be saturations of porous rock with a 
mineral solution or vapor from which ore has been deposited. ‘The cinnabar 
which is sometimes found impregnating unchanged or metamorphosed sandstone 
is generally cited as affording typical examples of impregnations. In such cases, 
which occur in California and South America, the deposit of ore has been 
ascribed by some writers to vapors, by others to solution, and it would seem that 
the latter is the more credible theory, although the vaporization of mercury is 
easily effected, and, like other metals, it may be transported by steam, as we 
have proof at the geysers in California. More familiar and satisfactory exhibi- 
tions of impregnation are, however, afforded by the copper-bearing sandstones of 
Lake Superior, New Jersey and New Mexico, and the silver-bearing sandstones 
of Silver Reef, in Southern Utah. In all these cases, it is evident that a porous 
rock was once saturated with a metalliferous solution, from which, in the Lake 
Superior region, metallic copper was precipitated; in New Jersey and New 
Mexico, sulphides of copper and iron; at Silver Reef, sulphide of silver. As 
such repositories of the metals are easily penetrated by surface water and air, we 
