84 abstracts: geology 



It may be stated, as a result of experience, that on clear individual 

 grains measuring from 0.01 to 0.03 mm. in diameter, all the optic prop- 

 erties ordinarily employed in the petrographic microscopic investigation 

 of minerals in the thin section can now be determined with a satisfac- 

 tory degree of accuracy. 



The introductory chapter contains a statement of the general theory 

 of the microscope and the uses of its individual parts and accessories. 

 Chapter I includes the application of the microscope in the determina- 

 tion of properties of the first class (color, pleochroism, absorption, 

 crystal habit, optical, character, dispersion, etc.), which do not admit of 

 numerical measurement. The properties of the second class which 

 admit of accurate determination and numerical expression are treated 

 in the following chapters: Chapter II, Refractive indices; III, Birefrin- 

 gence ; IV, Extinction angles; V, Optic axial angles. F. E. W. 



GEOLOGY. — Geology of the Berners Bay region, Alaska. Adolph 

 Knopf. Bulletin U. S. Geological Survey No. 446. Pp. 58, with 

 2 maps. 1911. 



Berners Bay is a broad and deep indentation from Lynn Canal, in 

 latitude 58° 42' north and longitude 135° west. The region under con- 

 sideration, whose areal extent is 50 square miles, embraces the long, 

 tapering peninsula and its mountainous background that lies' between 

 Berners Bay and Lynn Canal, and is at the northwestern extremity of 

 the long zone of auriferous mineralization on the mainland of south- 

 eastern Alaska, known as the Juneau gold belt. 



The Berners formation of slates and graywackes occupies the larger 

 part of the area and is well exposed on both sides of Berners Bay. The 

 strata show locally intense plication, but as a rule strike northwest and 

 southeast and dip steeply northeast. Fossil plants, consisting chiefly 

 of ferns, are indicative of Jurassic or Lower Cretaceous age. The older 

 view that the rocks of the Juneau gold belt are Carboniferous therefore 

 needs modification; it is probable that the rocks of the productive part 

 of the velt are mainly late Mesozoic. 



Northeast of the sedimentary rocks is a belt of much altered lavas, 

 chiefly basaltic amygdaloids. Northeast of these is an intrusive quartz 

 diorite gneiss, which constitutes the crushed and foliated margin of the 

 granitoid core of the Coast Range. 



The most important rock in the region from an economic point of view 

 is the Jualin diorite, which invades both the sedimentary and volcanic 

 rocks. The main ore bodies lie within its area. These are auriferous 

 deposits of low grade, most of which are well-defined fissure veins or 



