Page:Quarterly Journal of the Geological Society of London, vol. 26.djvu/260

158 hard chalk of 125*5 feet, the remaining 78 feet of basalt being hard, compact, and columnar. No iron-ore is exhibited in this series.

4. Metamorphism of Bole, and origin of the Pisolitic ore. — The spheroids of iron-ore are powerfully attracted by the magnet ; and the following analysis by Dr. Apjohn proves that the iron is in the states of per- and protoxides. The source of the iron is to be sought for in the hydrous peroxide of the bole, originally in combination with the silicated minerals of the basalt, and probably also in the hydrous oxide resulting from the decomposition of included magnetite and iron pyrites.

Analysis of Spheroids from Pisolitic ore, Red Bay.

Magnetic oxide of iron 11.74

Peroxide of iron 51.37

Silica, alumina, &c. (by diff. ) 36.89

100.00

Percentage of metallic iron 44.45

If we compare the analyses of basalt and ochre (given on p. 156), with those of the iron-ores (p. 159), we shall not fail to notice the general similarity between them ; the chief differences observable are the proportions of the several constituents, and, in the case of the pisolitic iron-ore, the different degrees of oxidization of the iron. But these differences do not militate against the presumed origin of the bole and iron-ore — that they are several stages in the metamorphism of one original mass : the addition of water to the basalt, and loss of lime, other alkaline earths, and alkalies, produced bole ; the loss of water and oxygen from the ferruginous materials of the bole resulted in an increased percentage of iron in the pisolitic ore. Though aqueous agency may have effected the first change, it could not have produced the second ; and heat is the force we must call to our aid in the alteration and concentration of the oxides of iron ; and by it the phenomena exhibited by the iron- band are then easy of explanation. We assume that the underlying bole or ochre was a wet or sodden terrestrial surface ; in fact many circumstances necessitate such a condition; and the subsequent overflow of basalt effected, by its heat, pressure, and possibly evolved gases, a reduction of the contained oxides of iron into the more concentrated form in which they occur in the iron-band, the aggregation of the ferruginous particles being a result of the same action *. This metamorphism is analogous to the development of new minerals by the intrusion of igneous masses among schistose and stratified rocks. It will be observed that the accumulation of the oxides of iron to certain centres diminishes as we recede from the exciting cause.

large number of the spheroids from the pisolitic bed, but once only found diatoms of the Coscinodiscus type, and not of the genera which form the bog-ore : they might have got there accidentally.
 * Since the reading of this paper before the Society, we have examined a