GOLD DEPOSITS. 61 
These ore bodies are of generous proportions, ranging from 40 or 50 to hundreds of feel in 
length and from 20 to several hundred feet in width. Owing to insufficient development 
up to the present time their depth has not yet been definitely ascertained. Some of the ore 
bodies persist below present workings, which are several hundred feet deep, while others 
appear to give out at a depth of 100 feet or more. These ore bodies are not tabular and of 
irregular extent, as might be expected from the form of the rock masses in which replace- 
ment has occurred; they are instead large, rudely lenticular or elliptical bodies, whose longer 
dimensions correspond with the structure of the replaced rock. The reason for this peculiar- 
ity of form is not clear. The most plausible explanation seems to be that the deposition of 
ore was greatly influenced by the presence of cross fissures which contained either ore 
solutions themselves or different solutions which, mingling with the ore-bearing solutions near 
the intersection of the two channels, caused a precipitation of the ore. Cross fissures which 
may have accomplished such a result are known in some places — for instance, at the Haile 
and Colossus mines. At the Haile they are commonly supposed to have had an effect on 
the deposition of the ore, but in reality it seems probable that these particular fractures are 
later than the ore. The question of the form and extent of these ore bodies is therefore not 
wholly settled. 
DEPOSITS OF INTERMEDIATE CHARACTER. 
As has already been stated, all the gold deposits give evidence of a common origin. It 
would therefore be surprising to find among these deposits a sharp division into the two 
classes just described. As a matter of fact, the division is not sharp. The two types are 
in reality the end members of a series in which nearly all intermediate gradations are to 
be found. Few, if indeed any, of the quartz fissure veins fail to show some replacement 
of their wall rocks; and at the other extreme are the narrow quartz veinlets which repre- 
sent the easiest channels for the passage of solutions and which gradually fade into the 
large siliceous replacement bodies. It is as unwise as it would be difficult to divide this 
series into a large number of types dependent on the amount of replacement along the 
veins. A few deposits, however, which seem to stand at about the middle of the series, 
are worthy of brief comment. The ore body of the Ferguson mine, in northwestern York 
County, is a good representative of this class. At that mine an interfoliated quartz vein 
penetrates a schistose volcanic tuff. Probably by the metamorphism and induration 
which accompanied foliation, this rock was rendered more resistant to attack by ore-bear- 
ing solutions than in most localities, but some alteration along the walls of the vein has 
taken place and has resulted in the production of a considerable zone of low-grade ore on 
each side of the richer quartz vein. The Blackmon, Kings Mountain, and Thompson 
deposits also appear to resemble in some respects both extreme types. 
MINERALOGICAL CHARACTER. 
The mineralogy of these gold ores is, on the whole, comparatively simple, but in some 
instances minerals rare in this association are present in considerable number. Owing to 
the different conditions under which they were deposited in the fissure veins and in the 
replacement bodies, the ores from these two sources exhibit certain mineralogical differ- 
ences. But since the veins in nearly all cases exhibit some metamorphic influence on the 
adjoining rock, and since the two types of deposits are in a broad way closely related, it 
would be unwise to attempt to make a fine distinction between the vein minerals and the 
replacement minerals. 
LIST OF MINERALS. 
A list of the minerals known in this area which constitute or accompany the gold deposits 
is here given, together with the name of the mine where they are found and a statement as 
to whether they are original constituents of the fissure filling or products of replacement 
in the surrounding rocks. Names printed in italic indicate decomposition products. 
Those original components of the wall rocks which seem to have been unaffected by the 
ore-bearing solutions, such as feldspar, amphibole, etc., are not included here. 
