326 abstracts: geology 



of greater extent than they are popularly supposed to be. The rock is 

 very much like the brown rock phosphate of Tennessee and belongs to 

 the same geologic epoch. Some specimens of Kentucky phosphate show' 

 a very high content of phosphoric acid but the material will have to be 

 washed and graded before it can be used for the manufacture of acid 

 phosphate. W. H. W. 



GEOLOGY. — The Bonnifield region, Alaska. Stephen R. Capps, Bul- 

 letin U. S. Geological Survey No. 501. 1912. Pp. 62, with maps, 

 views, and sections. 



The Bonnifield region lies on the north slope of the Alaska Range and 

 is bordered on the east by Delta River and on the west by the Nenana, 

 and includes both the rugged mountains of the main range and the foot- 

 hills belt. 



The northern part of the area is occupied by Birch Creek schist com- 

 posed of highly-contorted and fissile mica and quartz schists and phyl- 

 lites. The Totatlanika schist lying north of the Birch Creek schist in 

 the western part of the region is characteristically a porphyritic schist or 

 augen gneiss, with phenocrysts of feldspar and quartz in a fine-grained 

 groundmass composed chiefly of mica and quartz derived from rhyolites 

 or rhyolite porphyries, with perhaps some tuffs. Associated with these 

 altered igneous rocks are carbonaceous shales, schists, limestones, and 

 conglomerates of sedimentary origin. The Totatlanika schist is pro- 

 visionally assigned to the Silurian or Devonian. 



Eocene coal-bearing beds occupy warped basins in the schists and 

 underlie more recent deposits in the lowlands. 



The coal-bearing series is succeeded by the thick Nenana gravels. 

 These in places are conformable with the coal series but in other localities 

 there was a period of deformation and erosion between the two. The 

 gravels are believed to be Tertiary. 



The two schist series are cut by extensive bodies of intrusive rocks, 

 chiefly granites and diorites, and the intrusive masses were important in 

 causing the mineralization ofthe schists. Placer gold, derived originally 

 from the schists, and scattered in small quantities through the Tertiary 

 gravels has in places been reconcentrated into workable placers. 



S. R. C. 



