A composition quite sufficiently near to that of a basic kind of greenstone or basalt for us to look upon it as one.[1]
I shall not venture to intrude into the province of the chemist so far as to speculate on the way in which these additions were given to the rock, except by observing that the rock penetrates coal, and that the three ingredients acquired, namely, carbon, hydrogen and oxygen, are the essential constituents of coal.[2] Neither will I attempt to decide whether the infusion of these materials into the mass of the rock took place immediately on its injection into the coal, and while it was cooling and consolidating, or whether it was the result of the slow and gradual process of purely aqueous infiltration during long subsequent periods.
Basalt.
Rowley Hills—Of the basaltic rocks, the largest exhibition is that on the summit of the Rowley Hills. A capping of columnar basalt here rests upon the Coal-measures, over an irregular space two miles long by more than a mile in width at one part. This capping seems to be of irregular thickness, as it is largely quarried at some places on the side of the hills at least 100 feet below their summits; while on the line of the new Netherton tunnel of the Old Birmingham Canal no basalt was met with in any of the shafts, not even in the highest, which is very little below the crest of the ridge.
These shafts, however, seem to have passed through some pale green rock, slightly schistose and ashy-looking, with a crystal of feldspar here and there in it. There are also to be seen occasionally, on the slope of the hill just below the basalt, some considerable beds of trappean breccia, or brecciated ash, containing
- ↑ M. Durocher, in his Essay on Comparative Petrology, gives the following as the mean composition of Basalt and Diorite, of which latter term we may consider Greenstone as the English synonym:—
Diorite. Basalt. Silica, with trace of Titanic Acid 53.2 48.0 Alumina 16.0 13.8 Potash 1.3 1.5 Soda 2.2 3.0 Lime 6.3 10.2 Magnesia 6.0 6.5 Oxides of Iron and Manganese 14.90 13.8 Loss by ignition 1.0 3.2 100.0 100.0 (Essay on Comparative Petrology by M. J. Durocher, translated from the Annales des Mines, vol. xi., 1857, by Rev. S. Haughton. Dublin, McGlashan and Gill.)
- ↑
According to Bischof, bituminous coal may be taken as essentially composed of—
Carbon 82.1 Hydrogen 5.5 Oxygen and Nitrogen 12.4 100.0 independent of a variable admixture of earthy matter forming the ash.
