SHOSHONE MESA. 617 



grains of olivine, which can hardly be distinguished from the magnetite 

 grains, which have also a reddish, decomposed surface. Between the crys- 

 tals, particularly those of plagioclase, is an amorphous base of the usual 

 basaltic character. The quartz-grains are not seen under the microscope, 

 and are quite an accidental occurrence, probably caught up by the basalt 

 from the surrounding rhyolite. 



The main rock of the mesa is a normal dolerite, very porous, often 

 containing vesicules as large as a filbert, which are partially filled by carbon- 

 ate of lime. It is a dark, rather coarse-grained rock, in which plagioclase, 

 augite, and olivine can be distinguished by the naked eye, and, by the aid 

 of the microscope, also magnetite and some apatite, with a little amorphous 

 base. Chemical analysis detects the presence of titanic acid, so that some 

 titanic iron is probably associated with the magnetite. The analysis was 

 made by Mr. R. W. Woodward, and gives the following constituents: 



Silica , " 48.40 48.38 



Alumina,.,.,., .17.95 18.95 



. Ferric oxide 2.28 2.12 



Ferrous oxide 8.85 8.80 



Manganous oxide trace trace 



Lime 10.05 10.32 



Magnesia 6.99 7.02 



Lithia trace trace 



Soda : 2.86 2.73 



Potassa 1.03 1.03 



Titanic acid _ 0.24 0.24 



Carbonic acid . , .' , . , , , 0.84 — . 



Water - 0.34 ' 1.09 



99.83 100.78 



