478 



rROCEEDINGS OF THE NATIONAL MUSEUM. 



VOL. 37. 



and magnetite could be accurately determined. No allowance was 

 made for the small amount of alteration products present, for the 

 magnetite being in part ilmenite, nor for a slight proportion of ortho- 

 clase doubtless associated with the plagioclase. The error arising 

 from these sources was not considered sufficient to materially affect 

 the result. The average size of grain was found to be 0.17 mm. 

 and a total distance of 32.30 mm. was traversed, with the results 

 given in the table: 



Mineral com position or mode of olivine-duihasc. 



By giving definite values to the mineral components, the chemical 

 composition may be calculated from the mode. The plagioclase was 

 estimated to be Ab^Ang; to the augite was assigned the composition 

 obtained for this mineral in an olivine-hypersthene diabase from 

 Culpeper County, Virginia; '^ for olivine, an analysis of this mineral 

 from Montarville,'' Montreal, Canada, was taken. Magnetite was 

 given its theoretical value. The results are shown in the following 

 table : 



Chemical compositiott of olivlne-diabase calculated from its mode. 



CTiemical compositioji. — The chemical analysis of the olivine- 

 diabase is given in column I of the following table. Columns II 

 and III include the average compositions of olivine-diabase and nor- 

 mal diabase as obtained from a large number of reliable analyses: 

 In parentheses accompanying these two columns are shown the 

 limits for each component. The analyses of two related rocks are 

 added for comparison in columns IV and V. 



a Bull. Geol. Soc. Am., vol. 2, 1891, p. 344. 

 '> Amer. Jour. Sci., vol. 29, 1860, p. 283. 



