106 
ALASKAN MINERAL RESOURCES IN 1906. 
clearness the facts of possible economic importance. The rocks have 
been grouped as follows: 
1. Granitic rocks, chiefly quartz diorites. — These are probably intru- 
sive in rocks as high up in the stratigraphic column as the lower 
middle Jurassic. They apparently make up the main mass of the 
Talkeetna Mountains, and occur as isolated bosses on the south side 
20 miles 
_______ 
Tertiary sandstones, Jurassic volcanics, Slate-schist series Granitic rocks with 
shales and conglomerate, sandstones.shales, with intrusive granite areas of volcanics 
locally carrying coal and conglomerate, 
locally carrying coal 
Mica schist, 
gold bearing 
Fig. 2.— Geologic sketch map of region northeasl "f Cook Inlet. 
of Knik Arm. The importance of these granitic rocks lies in the 
possible mineralization which they may have induced in adjacent 
formations as a result of " eruptive after-effects." Such a causal 
relation between intrusion and mineralization has been recognized 
by Spencer a in the case of the Coast Range granites of southeastern 
Alaska and by Lindgren 6 in California. 
oSpencer, A. C, Magmatio OTigin of vein-forming writers in southeastern Uaska: Trans. Am. Inst. 
Min. Eng., vol. 36, 1906, p. 364. 
b Lindgren, W., Characteristics of the gold-quartz veins in Victoria: Eng. and Min. Jour., 1905, p. 460. 
