70 ALASKAN MINERAL RESOURCES IN 1904. [bull. 259. 
gold veins and in the general effects of mineralization, and some of 
the broader facts suggest that the dates of vein and ore deposition 
also correspond closely, though more extended and further detailed 
studies must be made before definite proof of this can be obtained. 
The formations of the mainland may be thrown into three lithologic 
groups, which are distributed in parallel zones following the general 
trend of the coast. Two of these groups, the schists and the slate- 
greenstone band, are mainly metamorphosed sediments, although the 
greenstone beds represent ancient volcanic flows of andesite and 
basalt. The third group is composed of the great complex of intru- 
sive granular rocks, mostly dioritic, which form the mass of the Coast 
Range. The general structure of the region is monoclinal, strikes 
being usually northwest and southeast and dips always toward the 
northeast. 
Very general mineralization has taken place since the diorite-intru- 
sions, the age of which has recently been determined as later than 
early Cretaceous. a 
THE ORE BODIES. 
GENERAL FEATURES. 
The ore bodies consist mainly of mineralized albite-diorite occurring 
in the form of intrusive dikes in black slates, the structure of which 
they closely follow. These slates are metamorphosed shales in which 
both original bedding and slaty structure strike northwest and south- 
east (fig. 1) and dip about 50° on the average toward the northeast. The: 
ore-bearing dikes belong to a series of intrusions which appear inter- 1 
ruptedly along the strike for a distance of about 3 miles in a zone] 
approximately 3,000 feet wide. In the greater part of the intruded! 
area exposures are few, and only small dikes outcrop on the side! 
toward the center of the island. On this side the zone seems to be] 
irregularly limited, but next to the shore of Gastineau Channel the; 
border is defined by a heavy bed of greenstone running parallel with 
the slates and the intrusive dikes and dipping with them toward the 
adjacent channel. The mineralized dikes that constitute the knownj 
minable ore occur just beneath this greenstone, which thus constitutes] 
the hanging wall both of the intrusion zone and of the ore bodies.] 
Many of the dikes of albite-diorite away from the hanging wall have] 
been greatly altered and impregnated with pyrite, but workable ore] 
bodies have not }^et been discovered in them. 
The strike of the different rocks is regular in the main, and, being 
slightly oblique to the channel, the outcrops of the ore bodies recede] 
from the shore toward the northwest. The base of the greenstone 
"During the summer of 1904 Mr. C. W. Wright found Lower Cretaceous strata on Admiralty Island 
infolded with slates and greenstones belonging to the same belt as those of Douglas Island. The 
diorites, which invade the slate-greenstone group of rocks, are either younger or of the same date asl 
the folding and are therefore younger than the Lower Cretaceous beds. 
