ABSTKACTS: GEOLOGY 225 



The detailed structure of the district is largely unknown, but in general 

 the Paleozoic rocks are severely deformed by folding and are metamor- 

 phosed; the Triassic rocks are strongly folded, the Jurassic rocks mod- 

 erately flexed, and younger formations are flat or only slightly inclined. 

 Triassic rocks are locally thrust upon Upper Jurassic rocks and normal 

 faulting has affected all pre-quaternary systems. The principal struc- 

 tural trend is northeast and southwest. There were great abysmal intru- 

 sions during the Lower or Middle Jurassic, and there has been volcanic 

 activity in Triassic, Lower Jurassic, Upper Jurassic, Tertiary, and Re- 

 cent times. 



The entire region was glaciated during the Quaternary, and a few 

 small glaciers still persist. 



The principal copper prospects of the region are in limestones at or 

 near the contacts of greenstone and of granite. Others are in quartz 

 veins traversing greenstone and granite. The silver prospects are man- 

 ganiferous sphalerite-galena veins in limestones. The gold prospects 

 are in pyritized fracture zones in granite and in quartz-latite tuffs, 

 and these are reported gold placers. Petroleum seepages occur along 

 the outcrop of the Middle and Upper Jurassic rocks. G. C. M. 



GEOLOGY.- — Geology arid coal fields of the lower Matanuska valley, 

 Alaska. G. C. Martin and F. J. Katz. Bulletin U. S. Geological 

 Survey No. 500, pp. 98 with maps, sections, and views. 1912. 



The coal field, situated in the lower part of the Matanuska valley, in 

 the south-central part of Alaska, is one of the most important in Alaska. 

 About 200 square miles of this region, including the larger part of the 

 known coal field, has been mapped on a scale of 1 : 62,500, while adjacent 

 areas have been covered by reconnaissance surveys. 



The stratigraphic sequence within the area studied in detail includes 

 Lower Jurassic, Lower (?) and Upper Cretaceous, Tertiary, and Quater- 

 nary deposits. There are also granitic masses of Mesozoic age, and intru- 

 sive diorite porphyries; trachytic rocks, and diabase and gabbro of Ter- 

 tiary age. In addition to these rocks there are, in the surrounding region, 

 Paleozoic or early Mesozoic schists, slates, graywackes, and greenstones, 

 Middle and Upper Jurassic shales, sandstones, and conglomerates. The 

 provisional lists of fossils include marine invertebrates from Lower, Mid- 

 dle, and Upper Jurassic and Upper Cretaceous horizons, together with 

 Tertiary and possibly LTpper Cretaceous plants. 



The great structural feature of the region is the fault or fault zone 

 which marks the straight south front of the Talkeetna mountains and 



