It is usual to speak of the molten material of the dykes as having risen vertically within the fissures. And doubtless, on the whole, the expression is sufficiently accurate. In the case of such long dykes as those of Central Scotland and the North of England, where the petrographical character of the material remains so uniform throughout, it is obvious that the andesite or dolerite cannot have come from a mere single pipe like a volcanic orifice. Nor can we easily understand how it could have been supplied even from a series of such pipes. The general aspect and structure of the dykes suggest that the fissures were rent so profoundly in the crust of the earth as to reach down to a reservoir of molten rock which straightway rose in them. The roof of such a reservoir, however, may have been irregular and uneven, so that a fissure need not have traversed it continuously, but may have only touched its upward projecting vaults. Hence gaps would arise in the continuity of the dyke-material.
The ascent of lava from a line of such separate openings along a fissure would necessarily involve lateral as well as vertical movements in the molten mass which would be forced along the open rent until the several streams united and filled it up. We might therefore expect somewhere to find instances of flow-structure in the dykes pointing to these movements. I have already referred to the lines of amygdales frequently noticed in dykes, especially towards the centre. Occasionally these steam-vesicles may be observed to be drawn out in one general direction indicative of the trend of motion of the molten rock.
Some of the best examples of this feature which have come under my observation occur among the trachytic dykes of the south-east coast of Skye between Kyle Rhea and Loch na Daal, where they have been mapped and carefully investigated by Mr. Clough, who has conducted me over the sections. In some of these dykes, as already narrated, the marginal portions display a finely spherulitic structure, the small pea-like spherulites being grouped into fine ribs or rods. It is also observable that the steam-vesicles which may retain their spherical forms in the centre are elongated in the same direction as the rows of spherulites. Where this lineation is developed vertically, it no doubt points to the vertical ascent of the lava between the two walls of the fissure.
But in other examples, the elongation is nearly horizontal, and between the two positions Mr. Clough has registered many intermediate trends. It would thus appear that in some places the lava has certainly flowed laterally between the fissure-walls. Moreover, the trend of the spherulitic rods and of the amygdales is found to vary in closely adjoining planes at different distances from the margin, as if after the outer portions of the dyke had consolidated into position, there was still movement enough to drag the rows of spherulites and vesicles up or down along the trend of the fissure.
Mr. Clough has observed that in some dykes, while the amygdaloidal vesicles are large and undeformed in the centre, they become elongated and inclined downward in the direction of the margin, as if the central portions had not only remained fluid longer than the rest, but had a tendency to rise upwards in the fissure, though there was obviously less motion after these central vesicles appeared than in the marginal parts where the vesicles are so much drawn out.
13. BRANCHING DYKES AND VEINS
Fig. 246.—Branching portion of the great Dyke near Hawick (length about one mile).
It might have been anticipated that the uprise of such abundant masses of molten rock, in so many long and wide fissures, would generally be attended with the intrusion of the same material into lateral rents and irregular openings, so that each dyke would have a kind of fringe of offshoots or processes striking from it into the surrounding ground. It might have been expected also that dykes would often branch, and that the arms would come together again and enclose portions of the rocks through which they rise. But in reality such excrescences and bifurcations are of comparatively rare occurrence. As a rule, each dyke is a mere wall of igneous rock, with little more projection or ramification than may be seen in a stone field-fence. Among the short, narrow and irregular dykes of the gregarious type branchings are occasionally seen, and in some districts are extraordinarily abundant. But among the great single dykes such irregularities are far less common than might have been looked for. A few characteristic examples from each type of dyke may here be given.
