OCR Text |
Show SYNTHETIC CHAEACTEB OF BASALT. 123 explain away without resorting to doubtful postulates and conjectures. Iron oxide forms at least 10 to 12 per cent, of normal basalt, and, while it is found abundantly in almost all foliated rocks, it cannot be admitted that it forms so large a percentage of their average constitution. With regard to lime, however, which forms about 8 or 9 per cent, of the basalts, the percentage is apparently in harmony with what we know of the constitution of the foliated rocks. With regard to the remaining important componentsâ€"magnesia, soda, and potashâ€"the same relative correspondence is found; but whether the correspondence be exact or not, we have not the data for determining. Belative order of abundance of the oxides constituting basalts and the foliated rocks. Basalts. Foliated rocks. Silica. Silica. "} r Silica. Alumina. Alumina Iron oxide. Lime. Lime. Magnesia, i Magnesia. Iron oxide. ) Soda. Soda. Potash. Potash. 3 ^or Alumina. Lime. Iron oxide. Magnesia. Soda. Potash. With the single exception of iron oxide, therefore, the basalts, as nearly as we have the means of ascertaining, have a constitution representing approximately the average composition and proportions of the foliated rocks. There is no other known volcanic rock which approaches that relation so nearly; all others contain too much silica and alkali and too little lime. But so long as the iron oxide remains an outstanding anomaly we cannot be justified in pronouncing the basalts to be the exact synthetic type. It remains to be added that the basalts alone fail to show that agreement in chemical constitution with any known and abundant metamorphic rock which we find in all other volcanic groups In truth, its whole range of characters is indicative of an origin among magmas which have never passed through the reactions and mechanical processes which prepared and arranged the materials of the sedimentary strata. Lastly, the basalts are among the most abundant of eruptive rocks, and if we reckon with them the more ancient dolerites or diabases, they have always been abundant in all ages as far back as our knowledge extends. |