38 abstracts: geology 



main geologic provinces — a Cretaceous basin to the east and an area 

 occupied by metamorphic rocks to the west. 



The Cretaceous rocks, which contain invertebrate fossils, cover a 

 large area and have a thickness of more than 20,000 feet, with neither 

 top nor bottom exposed. At the base of the Cretaceous section is the 

 Ungalik conglomerate, at least a thousand feet thick, which grades 

 upward, and probably laterally also, into the Shaktolik formation, of 

 sandstones and shales. The sandstone is made up of fairly fresh felds- 

 pathic and ferromagnesian minerals. Cross bedding, concentric weath- 

 ering, and mud-flat markings are common structures and some of the 

 beds are lignitic. 



The Ungalik and Shaktolik rocks are younger than the main igneous 

 intrusives, but in a few places they are cut by dioritic rocks, and the 

 mineralization effected by these has given rise to small placers. After 

 deformation and erosion the Cretaceous rocks were covered by basaltic 

 lavas. 



In the region of metamorphic rocks are schists, limestones, and 

 altered basic intrusives, all intricately folded and faulted. Fossils 

 from the limestones are Silurian to Devonian-Carboniferous. The 

 metamorphic rocks were intruded by granites and diorites after the 

 main period of metamorphism but before the deposition of the Cretaceous 

 sediments. 



The metamorphic rocks are those most mineralized, and there are 

 two series of auriferous quartz veins — one older and one later than the 

 period of metamorphism. Neither carries considerable sulphides. 

 These veins are the sources from which the placer deposits in the west- 

 ern part of the region, most notably near Council and Bluff, have been 

 derived. P. S. S. 



GEOLOGY. — The State Geological Surveys of the United States. Compiled 

 under the direction of C. W. Hayes. Bulletin U. S. Geological 

 Survey No. 465. Pp.177. 1911. 



Material compiled in cooperation with the administrative officers 

 directing State geological surveys. Thirty-six States now have organi- 

 zations for carrying on geological work. 



PALEONTOLOGY.— The fauna of the Moorefield shale of Arkansas. 

 George H. Girty. Bulletin U. S. Geological Survey No. 439. 

 Pp. 148, 15 Pis. 1911. 



The Moorefield shale is a member of the Mississippian section of north- 

 ern Arkansas lying above the Boone formation and below the Bates- 



