LODE MINING IN SOUTHEASTERN ALASKA. 81 
are the greenstones, a field name for highly metamorphosed and usually fine-grained rocks 
of a greenish cast, which range in composition from altered andesite and andesitic lull's to 
basalts and equivalent granular varieties. Intrusive dike rocks occur frequent Iv and have 
been injected at different periods. 
During the past summer special study was made of the fossil-bearing si rata of south- 
eastern Alaska by E. M. Kindle, in company with one of the writers. The results of this 
investigation show that the geologic column extends from the Silurian rocks upward. The 
Paleozoic formations constitute the greater portion of the sedimentary rocks, and are made 
up principally of limestones and schists. These strata have suffered in general such altera- 
tions that their fossil content has been effaced, and paleontologic evidence was rarely found. 
The Mesozoic formations, though folded and tilted on edge, are only slightly metamor- 
phosed and rarely altered to schist. Slates, graywacke, and conglomerates are the essen- 
tial rock types, though limestone was also observed. The area! distribution of the Mesozoic 
beds is relatively small. Tertiary beds have been deposited locally on Admiralty and Ku- 
preanof islands, and at these places are comparatively flat lying and unaltered. They arc 
made up of shale, sandstone, and conglomerate beds, and are coal bearing. 
ORE DEPOSITS. 
Taken as a whole the ore deposits of southeastern Alaska are of low grade, though there 
are many exceptions to this rule. Mineralization is widespread, metallic sulphides occur 
disseminated throughout most of the beds, but, concentration into workable deposits is 
relatively rare. 
The determination of the origin of the ore deposits is of direct commercial importance and 
it is unfortunate that the field studies have not progressed far enough to permit a solu- 
tion of the problem. While this phase of the subject, will not be discussed here, it can be 
stated' that the accumulation of observations bears out the suggestion made in previous 
reports" that there is a genetic relation between the ore bodies and the intrusives. 
GOLD. 
Gold occurs not, only in quartz ledges, but also in igneous dikes and altered bedded rocks. 
It is a frequent accessory constituent of copper deposits. The greater number of the ore 
bodies are low grade in character and are often passed over by the gold seeker, whose object 
is to find rich ledges or pockets of ore which will yield him quick returns with a small amount 
of labor. 
In 1905 placer gold was mined at only two localities— on Porcupine Creek,'' in the 
Skagway district, and on Gold Creek, in the Juneau district. 
JUNEAU DISTRICT. 
The mainland strip extending from Juneau to Berners Bay is doubtless the richest gold- 
bearing area in southeastern Alaska yet discovered. Developments within this mineral 
belt have made rapid progress during' the last year and future improvements will mate- 
rially increase its present gold production. 
BERNERS BAY. 
In the Berners Bay region members of the slate-greenstone series, together with intrusive 
granite and dionte belts, constitute the exposed rock types (PI. XI). The ('oast Range 
intrusive belt lies to the east, and branching from it is a narrow wedge-shaped mass of diorite 
which has invaded the sediments and is the locus of the principal deposits. At the head oi 
Berners Bay this diorite mass is 2 miles wide, while 3 miles north of Comet it is cut oil' by 
Lynn Canal and narrows to half a mile. 
o Brooks, A. IT., The Ketchikan mining district, Alaska: Prof. Paper U. S. Geo!. Survey No. 1, 1902. 
Spencer, A. C, The Juneau gold belt: Bull. U. S. Geol. Survey No. 225, 1903, pp. 28-42. 
b Wright, C. W., Porcupine placer district, Alaska: Bull. U. S. Geol. Survey No. 236, 1904. 
