The Shinumo Area. 513 



1. Sandstone _ 9' 4 



2. Arenaceous shale 21' 1" 



3. Sandstone - 9' 



4. Arenaceous shale 11' 8" 



5. Sandstone 2' 



6. Arenaceous shale 24' 



7. Sandstone 1' 



Total 78' 1" 



Synojosis of third member of the Unkar, 



a. Cliff-forming jasper — 73' 



b. Blue slate with calcareous band 18' 



c. Cliff-forming jasper — 17' 6" 



d. Sandy quartzitic jasper 52' 



e. Red and blue jasper. . 31' 



f. (Intrusive diabase) 



g. Blue slate and quartzite. 20' 



h. Blue slate . 10u' 



i. Red argillaceous shale 81' 



j. Alternating vermilion argillaceous 



shale and S. S 109' 4" 



h. Alternating vermilion arenaceous 



shale and S. S. ...- 78' l" 



Total thickness 579'11" 



Thin sections were cut from several specimens of the 

 jaspers. The slides were unsatisfactory, however, because of 

 the exceedingly fine grain of the rock. The highest power of 

 the microscope revealed nothing more than an impalpable 

 silicified mud. A slide of the " quartzitic jasper " showed it 

 to have been a somewhat arkose sandstone indurated to a 

 siliceous quartzite. It was seen to be composed chiefly of 

 small rounded quartz grains about which secondary silica had 

 been deposited, lying in a fine arkose matrix made up of small 

 fragments of pink feldspar. A thin section was also made 

 from a specimen of one of the sandstone layers in the " alter- 

 nating argillaceous shale and sandstone" of division "j." 

 The rock proved to consist of small, well-rounded grains of 

 quartz, cemented by silica in the form of secondary quartz. It 

 is a pure, fine-grained sandstone. 



The metamorphic effects produced by the diabase sill 

 intruded at the horizon "/*" are seen in the lithologic section of 

 the third member given above. This metamorphic action is 

 manifested in three ways : 



1. Induration by silicification, — jaspers. 



2. Induration by baking, — slates. 



3. Decoloration, — red to blue and black. 



Am. Jour. Sci. — Fourth Series, Vol. XXIX, No. 174. — June, 1910. 

 34 



