NO. 1715. OLIVTNE-DIABASE FROM NORTH CAROLINA.— POGUE. 479 



Analyses of olivine-diabase and related rod's. 



I. Chemical analysis of olivine-diabase from near Fairmont, Davidson County, Nortti Carolina. A. S. 

 Wtieeler, analyst. 



II. Average and extreme values of 9 analyses of olivine-dial:)ase appearing in Washington's Tables. 

 Includes all analyses of olivine-diabase marked ''superior." From Chemical analyses of igneous rocks 

 published from 1884 to 1900, U. S. Geol. Survey, Prof. Paper No. 14, 1903; and The superior analyses of 

 igneous rocks from Roth's TalieUen. 1869 to 1884, U. S. Geol. Survey, Prof. Paper No. 28, 1904. 



III. Average and extreme values of 62 analyses of diabase appearing in Washington's "Tables. Includes 

 all "superior" analyses of normal diabase. 



IV. Chemical analysis of dialxise (auvergnose). Mount Ascutney, Vermont. W. E. IliUebrand, analyst. 

 From U. S. Geol. Survey, Bull. No. 148, 1897, p. 70. 



V. Chemical analysis of olivine-diabase from Pittsylvania Countv, Virginia. T. L. Watson, analyst. 

 From Amer. Geol., vol. 22, 1898, p. 87. 



Discussion of the cliemical composition. — Comparing first column I 

 of the above table with the results obtained in the preceding table, 

 it will be seen that the composition as derived by microscopic deter- 

 mination is in pretty fair accord with the more exact results of the 

 chemical analysis. The chief points of difference lie in the rather 

 high amounts of magnesia and lime present in the former. This 

 discrepancy must be attributed chiefly to the values assumed for 

 the variable components, augite and olivine; that the first was too 

 calcic and the second too high in magnesia. The microscopic analy- 

 sis is sufficiently accurate to be of value in classifying the rock, inas- 

 much as its position in the quantitative system is the same, whether 

 calculated from the cliemical or microscopic analysis. 



Turning now to a comparison of the chemical analysis of the rock 

 described (I) with the average composition of olivine-diabase (II) 

 and normal diabase (III), the following features appear regarding 

 the Davidson County olivine-diabase: Low in silica; high in alumina; 

 ferric iron low; ferrous iron fairly high; magnesia high; lime a little 

 high; alkalies a bit low. The rock is consequently extremely basic, 

 due chiefly to the large amount of olivine present. The high per- 

 centage of alumina indicates a very aluminous augite, low in calcium. 

 The olivine is judged to be fairly high in ferrous iron. By a com- 

 parison with the extreme values for each component given in paren- 

 theses the relations of the rock under discussion are still more accu- 

 rately brought out, particularly in regard to those components, i. e., 

 alumina and magnesia, which approach most closely the extremes. 



