After the explosions ceased, by which the vents were opened and the cones of debris were heaped up, heated vapours would in many cases, as in modern volcanoes, continue for a long while to ascend in the funnels. The experiments of Daubrée on the effects of water and vapour upon silicates under great pressure and at a low red heat, have shown how great may be the lithological changes thereby superinduced. It is improbable that where a mass of tuff and lava, lying deep within a volcanic vent, was thoroughly permeated with constantly ascending heated vapours, it should escape some kind of change. I am inclined to attribute to this cause the frequent conversion of the sandstones round the walls of the vents into quartzite. The most remarkable example of metamorphism within a vent which I have observed among the plateaux, occurs in the heart of the Campsie Fells, where, instead of forming a prominence, the neck is marked by a great hollow, measuring about a mile in length and half a mile in breadth ([Fig. 128]).[440] It is occupied mainly by a coarse tumultuous agglomerate, like that of other necks in the same district, but with a matrix rather more indurated, and assuming in certain parts a crystalline texture, so as to be at first sight hardly distinguishable from some of the surrounding andesites. Even in this altered condition, however, its included fragments may be recognized, particularly blocks of sandstone which have been hardened into quartzite. Numerous small veins of pink and yellow trachyte traverse the agglomerate, and are found also cutting the bedded andesites that encircle it.
[440] See Explanation to Sheet 31, Geological Survey of Scotland, par. 21 (1878).
Fig. 133.—View of Traprain Law from the south, a phonolite neck of the Garleton Plateau.
3. DYKES AND SILLS
Intrusive masses both in the form of dykes and of sills are of frequent occurrence in connection with the Carboniferous volcanic plateaux. From the variety of their component materials it may be inferred that these rocks belong to different ages of intrusion.
Dykes.—The great majority of the Dykes consist of trachyte or of andesite, resembling in lithological characters the material of the necks and doubtless connected with its uprise. There occur also dykes of diabase, basalt or dolerite. Some of the latter, especially those which run for many miles, cutting every rock in the districts in which they occur, and crossing large faults without deviation, are certainly long posterior to the plateau volcanic period. Whether the small inconstant dykes of more basic composition, found in the same districts with the trachytes, are to be looked upon as part of the volcanic phenomena of the plateaux, is a question to which at present no definite answer can be given. I shall have occasion to show that in the next volcanic period the lavas that flowed from the puys are more basic than most of those of the plateaux, and that they are associated with more basic dykes and sills. In Roxburghshire, where it is so difficult to distinguish between the denuded vents of the two periods, the dark heavy olivine-basalts and dolerites of the bosses may possibly belong rather to the later than to the earlier volcanic episode. And if that be their true age, the dykes of similar material may be connected with them. At the same time it must be remembered that the earliest eruptions of the plateaux were markedly basic, that many vents in the plateaux are pierced by basic intrusions, and that basic dykes may have been associated with the uprise of the same magma.
The dykes occur in considerable numbers and in two distinct positions, though these may be closely related to each other: 1st, among the rocks outside and beneath the plateau-lavas, or cutting these lavas; and 2nd, in and around the vents.
1. Among the rocks which emerge from under the Carboniferous volcanic plateaux, dykes are sometimes to be observed in considerable numbers. They may be compared to the far more extensive series connected with the Tertiary basalt-plateaux, like which they may have had a close relation to the actual building up of the successive sheets of andesite, trachyte and basalt that were erupted at the surface. They are particularly well developed in the Clyde plateau, where by extensive denudation they have been admirably exposed. I would especially refer to those that traverse the tract of red sandstones which underlie the volcanic series along the flanks of the great escarpments from Fintry to Strathblane and Dumbarton, and between Gourock and Ardrossan. These dykes have been dissected by the sea along both sides of the estuary of the Clyde and in the islands of Cumbrae. In these islands and in Bute they have recently been mapped in great detail for the Geological Survey by my colleague, Mr. W. Gunn, who has supplied me with notes of his observations on the subject, from which the following summary is compiled.
"There are at least four distinct groups of intrusive rocks in the Greater Cumbrae. The oldest of these is trachytic in character, and occurs both as dykes and sheets, which run generally in the same E.N.E. direction. The rock is usually pinkish in colour, sometimes grey or purplish. A specimen from the dyke of the Hawk's Nest, north of Farland Point, analyzed by Mr. Teall, was found to contain 11 per cent of alkalies, principally potash, while the percentages of lime and iron were very low. Sometimes these rocks are fine in grain with only a few porphyritic orthoclase crystals, though numerous small crystals of this mineral are found with the aid of the microscope. These red trachyte dykes are almost confined to the Upper Old Red Sandstone, rarely entering the overlying white Calciferous Sandstones, and never invading the plateau-lavas. They are therefore probably of early Carboniferous age.