76 ALASKAN MINERAL RESOURCES IN 1904. [bull. 259. 
lenses formed by alternate bulging and pinching of the intrusive mass. 
In places the structure of the slate follows these irregularities, while 
elsewhere there is local crosscutting. Pinching and swelling of the 
diorite is shown in both vertical and horizontal cross sections of the 
dikes, though in general it is to be noted that the variations are more 
frequent and the changes take place within shorter distances upon the 
dip than upon the strike. 
The greater frequency of the variations on the dip, which has been 
mentioned, may be due to faulting, for in the west end of the " Glory 
Hole" at the Tread well mine, and in one or two other cases under- 
ground, where observations have been less readily made, the ore bodies 
are offset by movement along surfaces which strike nearly parallel to 
the veins, but dip at a lower angle. A series of such faults would 
produce the effect of alternate swelling and pinching (fig. 3). 
Considerable work was done several years ago in prospecting adja- 
cent bodies of diorite, many of which are as thoroughly impregnated 
with pyrite as the developed ore bodies. 
So far as known, the gold values are 
mostly very low; and while mines may 
yet be discovered, explorations have not 
thus far revealed workable ores. 
The occurrence of the sulphide- 
bearing diorite which forms the Tread- 
well ore deposit has been described by 
Dr. G. M. Dawson, who visited the 
mine in 1889. This geologist believed 
the deposit to represent the upper por- 
fig. a.-weai sketch showing manner tion or " feather edge" of a granitic 
in which faults of low dip may displace . . i i i 
an inclined dike, giving appearance intrusion, probably contemporaneous 
of alternate swelling and pinching. an( j connected with the granites of the 
neighboring Coast Range. The structural relations presented by this 
view are entirely in accord with present observations, for while the 
rock can not be strictly classed as granite, neither can a large part of 
the rocks which form the core of the Coast Range be so classed, since 
their composition is usually dioritic. The diorite of the Douglas Island 
mines doubtless belongs to the Coast Range period of intrusion; and if 
the small dikes of basalt which are found from place to place throughout 
the region be excepted, it is the }^oungest of the bed-rock formations 
in the vicinity. At the time of its intrusion the rocks which now 
appear at the surface occupied a position deep within the shell of the 
earth (lithosphere); and while many masses of the Coast Range diorite 
were forced through to the surface, it is doubtful whether any of these 
particular dikes ever extended as far as the surface which then existed. 
Taken together they represent intrusive material which was arrested 
en route, while larger masses of related rocks in the region are; 
