Page:Quarterly Journal of the Geological Society of London, vol. 26.djvu/258

156 the hard and black basalt of the interior and the purple or yellowish argillaceous masses in which the basalt appears to be imbedded. Again, the boles often contain cavities fdled with the ordinary zeo- lites of the amygdaloid basalts. Portlock* gives an analogous case of the production of ochre from an allied rock. " The greenstone in the Glenrandal river in Upper Cumber," he says, "is so highly charged with iron as to disintegrate into extensive beds of a very rich ochre. The ochre is sometimes in the state of hydrate and yellow, and at other times red. The mode of alternation is : — felspathic mica-schist, or gneiss, compact below, but quite rotten and decomposed at the top ; ochre graduating into a decomposing greenstone. * * * * The green- stone also occurs within the ochre, and by its decomposition gradually merges into it. * * * In a branch stream to the west, the ochre is met with alternating with less altered greenstone, the thickness of the greenstone being 26 feet, and of the ochre 18 feet. And in the townland of Aughlish, in Banagher, it is upwards of 50 feet thick, several patches of greenstone within the mass being still solid, and merging into ochre."

Observations in the field and the following comparative analyses go far to prove that the bole and lithomarge are the resultants of aqueous action in combination with acidulated gases, which, dis- solving out certain mineral substances, has effected the decomposi- tion of the basalts, especially the more felspathic ones.

Table of Analyses of Basalt, Lithomarge, and Bole.

Silica Alumina Peroxide of iron Lime Magnesia Sulphuret of iron tr. Soda Potash 9.94 6.35 15.10 Water 4.30 4.48 19.60

I. Basalt, Slieve Gallion (Dr. Apjohn). II. Basalt, Antrim (Prof. Hodges). III. Lithomarge, Germany. IV. Ochre of the Basalt, Drumrankin, near Ballymena (Dr. Apjohn). V. Bole, Germany.

The chief differences in the chemical composition of bole, lithomarge, and basalt are the increased percentage of peroxide of iron and water, and the less quantity of alkalies in the boles and lithomarges as compared with the basalt.

The alternation of basalts and boles indicates successive lava-flows of the more easily decomposed and the crystalline materials, the former having subsequently disintegrated into bole or lithomarge, while the latter has persisted tolerably well. In those instances where there is a passage from the hard rock below to the soft, and from the soft to the hard above, the basalts overlying and underlying

I. Bas. 39.72 53.70 49.75 56.40 30.88

II. Bas. 14.32 25.41 29.88 3.46 20.76

III. Lith. 27.87 8.95 6.61 24.14 26.16

IV. Och. 4.15 4.55 0.43 0.90 2.60

v. Bole. 4.00 1.47 tr. 9.94 6.35 15.10

4.30 4.48 19.60


 * Geol. Survey, Derry, p. 173.