12 TIN DEPOSITS OF THE YORK REGION, ALASKA. [no. 229. 
Purpose of this bulletin. — It is the purpose of this bulletin to com- 
bine the results obtained by the United States Geological Survey parties 
that have visited the region, together with the information derived 
from a study of specimens of tin ores and associated minerals recently 
brought from the York region by outside parties, and to present such 
facts in regard to the occurrences and value of the metal as may be of 
assistance to those interested in the development of the field. Through- 
out the field and office work the writer has had the efficient aid of 
Mr. Hess, who has devoted special attention to the compilation of 
the literature referring to tin deposits. The work of Mr. Eugene 
C. Sullivan, chemist of the Survey, who elaborated a method of 
analyses by which minute traces of tin could be detected, and who also 
made assays of the material from the York region, has added greatly 
to the value of the report. 
GENERAL GEOLOGY. 
The geology of the York region, as has been shown, has been sub- 
ject to investigations during the years 1900, 1901, and 1903, but all 
of this work was of a reconnaissance character, and the results have 
not yet been correlated with the latest work in other parts of the penin- 
sula; hence it has been thought best to defer their publication for the 
present. 
A sketch map (fig. 2) is here introduced to show the relative distribu- 
tion of the more prominent rock types, without attempting, however, 
to subdivide them into formations or to indicate their stratigraphic and 
structural relations. In this map the horizontal distribution of four 
different rock types is indicated. These include slates and limestones, 
probably of Paleozoic age, and some granular intrusives, chiefly of a 
siliceous character. The slates and limestones form belts of irregular 
outline extending north and south, while the igneous rocks are found in 
intrusive stocks and dikes, the former outcropping in more or less 
circular areas. Besides these hard rocks, Pleistocene and Recent sands 
and gravels form the surface deposits of the northern coastal plain, 
and are also found in the valleys of many of the streams. 
SEDIMENTARY ROCKS. 
It is seen in fig. 2 that the larger part of the area surrounding the 
York Mountains is occupied by limestone. This limestone has an 
ash-gray color and exhibits little evidence of metamorphism. It is 
characterized by low dips and comparatively simple structure. This 
formation has been called the Port Clarence limestone, a and has been 
definitely traced over an area of about 1,400 square miles, extending 
a Collier, A. J., Reconnaissance of the northwestern portion of Seward Peninsula, Alaska: Prof. 
Paper U. S. Geol. Survey No. 2, 1902, p. 18. 
