24 FORTYMILE, BIRCH CREEK, AND FAIRBANKS PLACERS, [bull. 253 
but whatever the conditions the resultant products have the earmarks 
of igneous origin. 
Shale, sandstone, and conglomerate, which are very common in the 
Seventymile Valley, are composed, on the other hand, of mud. 
rounded grains, and pebbles, respectively, arranged in rather well- 
defined layers that give the coarser varieties a banded appearance. 
All lines of evidence lead to the conclusion that such characters are due 
to the reworking and sorting of material derived from older rocks and 
its subsequent deposition by water. Except that they are consolidated 
and folded, these rocks differ in no essential respect from similar 
deposits now in process of formation. Limestones also are known to 
have been deposited under water and often contain recognizable 
remains of organisms. The unaltered igneous and sedimentary rocks 
thus show their mode of origin by their characteristics. 
The most widely distributed rock's within this area are the schists 
and gneisses, rocks which have been so changed by complex processes 
of metamorphism that their origin is often in doubt. The sedi- 
mentary and igneous rocks, in passing through the metamorphic mill. 
have had new minerals developed and new structures imposed upon 
them until their primary characteristics are obscured or replaced by 
those of a secondary nature. Further, these two classes, which are 
in places most intimately mingled, react on each other, and, beside 
the mutual results of such close relation, have often undergone 
together the processes of metamorphism. The formation of quartz 
veins is generally an accompaniment of these changes, while, locally, 
the quartz veins or the rocks themselves have been mineralized and 
become sources of some of the gold deposits, the discovery of which 
has made the region of importance and rendered the distribution of 
the metamorphic rocks a problem of economic interest. 
STRATIGRAPHIC SUCCESSION. 
An important purpose of all geologic work in any given province is 
to determine the sequence of the strata which outcrop in the area. In 
a region where the rocks have been much deformed the succession can] 
be definitely established only after the most painstaking and detailed 
studies, which in the Yukon-Tanana region have not yet been begun. 
The results of the various surveys have, however, determined the sue-] 
cession of the larger subdivisions of a geologic time scale, which id 
presented in the following table. This scale, which is practically 
identical with that prepared by Mr. Collier" with the cooperation 
of Mr. Brooks, will, in connection with the map (PI. IV), elucidate 
the general geologic features. 
" Collier, Arthur .T., The coal resources of the Yukon : Bull. TJ. S. Geol. Survey NoJ 
218, 1903, j). 15. 
