Page:The Journal of geology (IA journalofgeology11893univers).pdf/286
analyzed by Dr. H. N. Stokes of the laboratory of the U. S. Geological Survey. No. 8589 contains a large proportion of diallage and olivine, while No. 8786 is more nearly of the average composition of the entire mass.
| 8589 | 8786 | ||
|---|---|---|---|
| SiO2 | 45.66 | 46.45 | |
| TiO2 | .92 | 1.19 | |
| P2O5 | .05 | .02 | |
| Al2O3 | 16.44 | 21.30 | |
| Cr2O3 | tr. | ||
| FeO | 13.90 | 9.57 | |
| Fe2O3 | .66 | .81 | |
| NiO | .16 | .04 | |
| MnO | tr. | tr. | |
| CaO | 7.23 | 9.83 | |
| MgO | 11.57 | 7.90 | |
| K2O | .41 | .34 | |
| Na2O | 2.13 | 2.14 | |
| H2O at 105° | .07 | .14 | |
| H2O above 105° | .83 | 1.02 | |
| Total, | 100.03 | 100.75 |
The larger percentages of Al2O3 and of CaO in 8786 as compared with 8589, and the smaller percentages of FeO and MgO, substantiate the results of the microscopical study. An increase in the proportions of Al2O3 and CaO indicates an increase in labradorite, and a decrease in FeO and MgO, a decrease in iron-bearing minerals olivine and diallage. The variations are somewhat larger than was to be expected in a rock so uniform in structure and so monotonous in composition as that of this great mass, but they are easily accounted for by the local accumulation of certain of its heavier constituents. So far as known there are no "schlieren" in the normal rock nor any other evidences of a differentiation ("spaltung") of its magma before cooling, so that the variations in mineralogical and chemical composition must be looked upon as due purely to accidental causes. Moreover, the differences are not great enough to effect any material impression upon the rock as a whole. Its characteristics are practically identical throughout an area of several thousands of square miles, and are