The Shinumo Area. 509 



"When the specimens of these rocks were examined in the 

 laboratory it was found that all the limestones were more or 

 less dolomitic. The limestones of division "<?" were found to 

 be entirely dolomites. 



Thin sections were cut from specimens from twenty separate 

 beds in the second member. Eighteen of these slides were cut 

 from the limestone strata and two from the red shales. The 

 sections of the limestones were cut both from the chert 

 bands and the nodules and from the limestone itself, for 

 the purpose of ascertaining the exact mineralogical char- 

 acter of these rocks, and in the forlorn hope that they 

 might reveal traces of a structure that could be referred 

 to something organic. No minerals other than calcite and 

 quartz were revealed by the microscope in any of the slides. 

 The silica of the chert bands and nodules was found to exist 

 in the form of interlocking grains of quartz. None of the 

 grains were rounded and there was no suggestion that the 

 quartz grains of the chert bands represented an inwashed sand. 

 Nor was there any trace of an organic structure revealed, 

 either in the chert or in the limestone. The purer limestones 

 were found to consist of calcite (or dolomite) alone, the crys- 

 talline forms having the typical structure of marble. The 

 impure varieties were found to consist of mixtures of quartz 

 and calcite in all proportions. The greater part of the lime- 

 stone was of this impure character. The shales were found to 

 consist of a fine impalpable ferruginous or calcareous mud, 

 containing occasionally a minute grain of quartz. 



Several features of interest are shown in the lithologic sec- 

 tion of the second member of the Unkar as a whole. 



Ripple marks and sun cracks appear for the first time in the 

 shales in stratum No. 18 of division u d" just below the highest 

 limestone stratum at the summit of the member. 



The increasing intensity of metamorphic phenomena in the 

 section from the base upward may also be noted. This is due 

 to the approaching proximity to the lower contact of the dia- 

 base sill which is intruded in the member above. The 

 metamorphic action is manifested in the shales by their change 

 upward both in color and in degree of induration : the shales 

 of division " b " are almost entirely red ; from the summit of 

 this division upward the color changes to purple and blue. 

 Below division " <:Z" the shales are soft and crumbly; within 

 this division, however, they become dark blue slates, while in 

 the upper part they become extremely hard, siliceous jaspers. 



The vertical succession of the strata is seen to be broadly 

 characterized by continual and rapid alterations of limestone 

 and shale. According to the dominance of either type of rock 

 the four divisions &, 5, c, and d are separated : division " a " is 



