652 



ERUPTIVE BOCKS FROM MONTANA— MERRILL. 



mottled, deep briglit greeu cleavage plates (sometimes 5 to 10 mm. 

 across) of a mineral of the j)yroxeue group, and abundant small folia of 

 brown mica. To the unaided eye these two minerals form the chief con- 

 stituents of the rock. This variety weathers with peculiarily knobbed 

 and deeply pitted surfaces. 



The second variety differs only in being of finer grained and more 

 uniform texture, its mineralogical nature being barely evident even 

 with a pocket lens. 



Both varieties are, however, essentially the same. Under the micro- 

 scope the rock is found to be beautifully fresh and unaltered — a holo- 

 crystalline granular aggregate of pale green diallage, deep reddish- 

 brown mica, colorless olivines, rarely small irregular areas of a basic 

 plagioclase, and scattering patches and streaks of black iron oxides, 

 which are evidently wholly secondary. None of the constituents pre- 

 sent anything like perfect crj'stal outlines. 



The structure is peculiarly jumbled and almost cataclastic. It 

 resembles more the hasty and interrupted crystallizations character- 

 istic of certain meteorites, like that of Estherville, Iowa, than that of 

 terrestrial rocks. Diallages and olivines are crowded and jumbled 

 together, the interstices of the larger forms being occupied by the 

 same minerals in a granular condition. 



The diallage has reached the most perfect stage of developement and 

 often occurs in broad idates inclosing olivines and shreds of brown 

 mica, and with a very pronounced pinacoidal parting. Feldspars, 

 when they occur, are in short broad plates sometimes polysynthetically 

 twined or again showing broad faces without trace of cleavage or twin- 

 ing lines and filled with small colorless and yellow interpositions of 

 mica. (?) The prevailing mica is deep brownish-red and strongly pleo- 

 chroic. The following shows the composition of the rock according 

 to analysis by Mr. L. G-. Eakins, of the U. S. Geological Survey: 



SiOj . 

 TiOj . 

 Al.,03 

 Cr.Os 

 Fe.,03 

 FeO. 

 MnO. 

 NiC. 

 CaU . 



Per cent. 



48.95 



.81 



5. 69 



.05 



1.20 



12.11 



.08 



.16 



5.33 



MgO 



Bai ) 



K./) 



Ka.,() 



HjO 



PaOg 



Specific gravity 



Per cent. 



23.49 



Trace. 



.79 



1.58 



.18 



.12 



100. 54 

 3.37 



Diorites. — Burnt Creek region. These rocks are apparently all diorites 

 and presumably portions of the same geologic body, though differing 

 somewhat in composition and in details of structure. Some are fine 

 and evenly granular dark-gray rocks, showing under the microscope a 

 holocry stall ine, panidiomorphic granular structure, a portion of the 

 plagioclases only showing idiomori)hic developement. A green horn- 

 blende in very irregular plates and shreds is, next to the plagioclase, 



