Page:Quarterly Journal of the Geological Society of London, vol. 26.djvu/257

1869.] to its position relatively to that of the underlying White Limestone — which, by the way, is of varying thickness, and that often within very limited distances. At Red Bay the iron-ore is about 200 feet above the upper line of the White Limestone, whilst at Galboly, two miles to the east, in the direction in which the White Limestone thickens, it is about 300 feet vertically above that formation.

III. Origin of the Pisolitic Iron-ore.

1. Suggested Theories. — Four theories may be suggested to account for the origin of the iron-ore ; these are : —

(1) Sedimentary theory, which implies a derivative origin, but is at variance with the petological features — the distribution of the mineral particles has evidently been regulated by some other force than that of gravity. Mr. Du Noyer, in a paper on the Geology of Island Magee, read before the Natural-History and Philosophical Society of Belfast, November 25, 1868, endeavoured to demonstrate that the iron-ore in the basalt of Island Magee was entirely due to the action of water, " that it was as true an aqueous conglomerate as if it had been found in the heart of the Old Red Sandstone." From this opinion we most emphatically dissent, though at the same time we fully concur with him when he assigns such an origin to the iron beds at Ballypalidy.

(2) Theory of Deposition, implying either a precipitation of the ferruginous material from chemical solution, or segregation by organic agency. We believe that the state of combination of the oxides of iron, the intimate structure of the spheroids, and the petrology necessitate some other explanation.

(3) Igneous theory, implying production by direct volcanic action. Though specular iron is enumerated among the minerals emitted from active volcanoes, and magnetite and iron pyrites are accessory constituents of several of the basaltic strata, yet such an origin does not meet the requirements of the case in point. The distribution of the spheroids of pisolitic ore demands either a shower of the ferrous nodules over a large area, the smaller ones falling before the larger, or a flow of volcanic mud with suspended spheroids, the smaller sinking first in the mass — both assumptions being highly improbable.

(4) Metamorphic theory. — It is only by metamorphism that all the phenomena connected with the pisolitic ore can be interpreted. By metamorphism we understand something more than the effects of heat, and employ the term in its widest significance. We will now proceed to point out what appear to have been the several stages of metamorphic action by which the pisolitiec ore has been elaborated out of basalt.

2. Origin of Bole and Lithomarge. — There can be no doubt that the lithomarge and bole alternating with compact basalts are but decomposed basalts. The passage from basalt to lithomarge and bole can be traced in all cases ; not unfrequently large masses of basalt are to be seen in the midst of the lithomarge, the concentric layers presenting all the varieties of texture and colour between