REVIEWS 199 



The Iditarod-Ruby Region, Alaska. By Henry M. Eakin. Bull. 

 U.S. Geol. Surv. No. 578, 1914. Pp. 45, pis. 6 (including 4 

 maps), fig. 1. 



The Iditarod-Ruby region is situated in west-central Alaska between 

 the headwaters of the Iditarod and the Yukon at Ruby. 



The geologic succession is as follows: probable Paleozoic meta- 

 morphic rocks; Cretaceous sedimentary and volcanic rocks; post- 

 Cretaceous intrusives; Quaternary unconsolidated deposits that include 

 glacial material. 



Conglomerates (in places several hundred feet thick), the material 

 of which has been derived from the underlying metamorphic rocks, 

 occur principally near the base of the Cretaceous beds. Some contain 

 bowlders up to 3 feet in diameter. 



Placer gold, with a minor amount of silver, is the mineral resource 

 of the region. The gold has been derived chiefly from quartz veins 

 (which are probably genetically related to the post-Cretaceous intru- 

 sions) that traverse the igneous, sedimentary, and metamorphic rocks. 



The value of the gold and silver produced in 191 2 in the Iditarod, 

 Innoko, and Ruby districts was respectively $3,500,000, $250,000, and 

 $150,000. 



In 1 9 13 the value of the winter production in the Ruby district was 



$102,200, while that of the summer production was estimated at 



$750,000. 



V. 0. T. 



The San Franciscan Volcanic Field, Arizona. By Henry Hollis- 

 ter Robinson. Professional Paper, U.S. Geol. Survey, No. 

 76, 1913. Pp. 213, pis. 14 (including 2 maps), figs. 36 (includ- 

 ing 8 maps). 

 The San Franciscan volcanic field embraces an area of about 3,000 

 square miles in north-central Arizona. 



Chap, i is devoted to the geography of the region. 

 Chap, ii treats chiefly of the sedimentary formations and structure. 

 The sequence of sedimentary rocks is as follows: the Mississippian and 

 Pennsylvanian Redwall limestone; the Pennsylvanian Supai formation 

 ("Lower Aubrey" sandstone and shale), Coconino ("Upper Aubrey") 

 sandstone, and Kaibab ("Upper Aubrey") limestone; the Permian(?) 

 Moencopic formation (red to light-brown shales, with some sandstone 

 and calcareous layers); the Triassic " Lithodendron formation" (basal 



