One of the most interesting features of this vent is to be found in its relation to the surrounding basalts. The marginal parts of the rock along the line of contact are much finer in grain than the rest, and have obviously cooled more rapidly. The contrast between them and the ordinary dolerite nearer the centre, however, cannot be properly understood, except in thin sections under the microscope. Dr. Hatch, to whom I submitted my specimens, observed that, in place of the structure above described, the marginal parts show an absence of the ophitic grouping except in small isolated patches. Instead of occurring in large grains or plates enveloping the felspars, the augite is found in numerous small roundish grains, together with grains of magnetite, in equal abundance and of similar size. The felspars are speckled over with opaque particles; olivine has not been detected.
For miles around the vent, the plateau-rocks are of the usual type—black, compact, sometimes amygdaloidal, alternating with more coarsely crystalline decomposing bands, the separation between different sheets being often marked by the ordinary red ferruginous partings. But around the margin of the neck, they have undergone a remarkable metamorphism. The portions of them which adhere to the outer wall of the neck have lost their distinct bedding, and have been, as it were, welded together into an indurated compact, black to dull-grey rock, so shattery and jointed that fresh hand-specimens, three or four inches in length, are not easily obtainable. Especially marked is one set of joints which, running approximately parallel, cause the rock to split into plates or slabs. These joints are sometimes curved. Yet, in spite of the alteration from its normal character, the basalt retains in places some of its more usual external features, such, for instance, as its amygdaloidal structure, the amygdales consisting of calcite, finely acicular mesotype, and other minerals.
Examined under the microscope, this altered basalt presents "a confused aggregate of colourless microlites (felspar?) and innumerable minute granules of magnetite, these two constituents being very unequally distributed. Sometimes the colourless portions preponderate, in other places the opaque granules are heaped together in black patches, which may possibly mark the position of fused augites."[297]
[297] Notes by Dr. Hatch.
In the zone of contact-metamorphism around some of the volcanic pipes in the plateaux, we see changes analogous to, but less developed than, those which have been superinduced on so large a scale round the great eruptive bosses of gabbro, granophyre, etc., that have broken up the terraced basalts along the west coast of Scotland. I shall accordingly return to this subject in connection with phenomena presented by these younger rocks ([p. 386]).
ii. VENTS FILLED WITH AGGLOMERATE
While the necks of dolerite or basalt cannot always be satisfactorily discriminated from bosses which may never have established a connection with the surface, there is no room for any doubt in this respect in the case of those filled with fragmentary materials. As has been already pointed out, the occurrence of true volcanic agglomerate may be accepted as evidence of the existence of an eruptive vent communicating with the surface of the earth. The agglomerate in the vents associated with the basalt-plateaux, like that of the Palæozoic vents, is generally exceedingly coarse, and without any trace of structure. Blocks of all sizes up to masses some yards in length, and of the most diversified materials, both volcanic and non-volcanic, are dispersed confusedly through a granular paste of similar miscellaneous composition.
Fig. 301.—Diagram to show the probable relation of the Neck at Carrick-a-raide, Antrim, to an adjacent group of tuffs.
a a, Chalk; b b, lower group of bedded basalts; c, vent of Carrick-a-raide, filled with coarse volcanic agglomerate; d d, bedded tuffs; e e, large veins of basalt traversing the agglomerate; f f, zone of tuffs and pisolitic iron ore; g g, upper group of bedded basalts.
An instructive example of the general characteristics of agglomerate-vents, and of the relation of these vents to the surrounding tuffs and basalts, is to be found at the island of Carrick-a-raide, on the north coast of Antrim, and on the opposite mainland. The visible mass of this neck is about 1000 feet in diameter, but the boundaries, except on the land side, are concealed by the sea. The material filling up the vent is a coarse agglomerate, in which blocks and bombs of basalt, with pieces of chalk and flint, are stuck at all angles in a dull dirty-green granular tuff. Some large and small intrusions of basalt rise through it. Owing partly to these intrusions, and partly to the grass-covered slope that separates it from the line of cliff, the actual contact of this neck with the volcanic beds of the escarpment cannot be seen. I have no doubt, however, that the tuff, which has already been referred to as so conspicuous a member of the series here, was discharged from this vent.[298] The materials are as usual coarser in the pipe than beyond it, but the finer portion or matrix of the agglomerate is similar to many bands of the tuff. The structure of the locality may be diagrammatically represented as in [Fig. 301]. The bedded tuff is thickest in the neighbourhood of the vent, and gradually dies away on either side of it.