pkindlk.1 GEOLOGIC SKETCH. 23 
from the plateau country across the flats to Tanana River is about 
8 miles. The Chena Slough, however, from the main river, mean- 
ders alone: the northern limit of the flat and receives the streams soon 
after they leave the hills. The altitude of the slough at Fairbanks is 
approximately 450 feet. 
GEOLOGIC SKETCH. 
INTRODUCTION. 
The shape and surface of the area have been briefly described, and 
the material or bed rock of which this block of country is composed 
next demands consideration. The attainment of detailed knowledge 
of a region of complex geology like that under discussion is the work 
of many years, but the results from the various reconnaissance sur- 
veys have thrown much light on the broader geologic problems, and 
these are epitomized in the table of stratigraphic succession to follow, 
as well as on the map (PL IV). 
Both in the field and in the laboratory the writer's chief energies 
have been directed toward a study of the economic problems, to which 
the purely geologic investigations have been made entirely subor- 
dinate. In view of this and because more detailed studies of the 
stratigraphy are to be undertaken, only a few general conclusions in 
regard to the geology will be here presented. The stratigraphic 
succession of the general province will be presented in tabular form, 
but in the text only those formations and rocks which the writer 
has personally studied will be considered, and these chiefly from the 
standpoint of economic importance, the constant aim being to present 
such matter as will enable the miner to understand better the relation 
of the deposits in which he is interested to the bed rock in which 
they occur, or from which they have been derived. 
The rocks include representatives of the igneous and sedimentary 
classes — those clearly formed by solidification from a molten condi- 
tion and those which have been deposited through the agency of 
water — and the metamorphic rocks — a class most important in this 
region — which result from the alteration of either of the others. 
Granite, which occurs abundantly in this area, illustrates the first 
class.* The minerals of which it is composed have developed in place 
as the final result of the slow cooling to which the rock material has 
been subjected. With rapid cooling there would have been little 
opportunity for the development of minerals, and the molten mass 
would have consolidated as a product somewhat resembling the slag 
from a glass furnace. Coarse-grained, fine-grained, and glassy rocks, 
entirely different in appearance, may thus result from the same mate- 
rial simply through a difference in the conditions of solidification; 
