648 



ERUPTIVE BOCKS FROM MONTANA— MERRILL. 



but is a true crystalline rock, the original fragments forming propor- 

 tionally large nuclei to a mass of crj^stalline granules whose regular 

 growth has been interrupted by mutual interference. How much or 

 how little of this change is due to the injected rock it is impossible 

 to say. 



Chemical analysis of as fresh a sample of the eruptive as was obtain- 

 able from near the central portion of the sheet yielded Mr. Eakins 

 results as follows : 



SiOj . 

 TiOs- 

 AI2O3 

 re,03 

 FeO . 

 MnO. 

 liaO . 

 CaO . 



Per cent. 



Per cent. 



52.33 

 .14 



15.09 

 4.31 

 4.03 

 0.09 

 0.07 

 7.06 



MgO 



K2O 



Na^O 



HjO. 



PyO, 



Specific gravity In mass 



6.73 

 3.76 

 3.14 

 2.68 

 1.02 



100. 45 

 2.785 



Considering all the potash in the above as belonging to the ortho- 

 clase and the soda to the plagioclases, these results can be reduced 

 readily to the following proportions: 



Such calculations must, of course, be accepted only with a consider- 

 able degree of allowance. It is probable that a portion of the potash 

 belongs to the plagioclase feldspars, and, without doubt, a small amount 

 to the mica, for which no allowance whatever has been made. This 

 last amount would, however, be trifling. A safer but less definite cal- 

 culation is as follows : 



The above, I believe, represents the proportional qualities of the vari- 

 ous constituents as nearly as it is possible to obtain them. 



To the west of the outcroppings of the sheet occur rather incon- 

 spicuous outcroppings of a darker, more compact rock with macroscopic 

 olivines and augites in macroscopically recognizable forms. (No. 38516, 

 U.S.N.M.) This is described in detail in connection with the basic 

 eruptive overlying the mica syenite between Antelope and South 

 Boulder creeks (p. 671). 



