Fig. 343.—Theoretical representation of the structure of one of the Gabbro Bosses of the Inner Hebrides.
a a, platform of older rock on which the bedded basalts (b b) have been poured out; c, gabbro.
The structure and history of the gabbro bosses of the Inner Hebrides find a close parallel in those of the Henry Mountains of Southern Utah, so well described by Mr. G. K. Gilbert of the United States Geological Survey.[357] In that fine group of mountains, rising to an extreme height of 5000 feet above the surrounding plateau, and 11,000 feet above the level of the sea, masses of trachyte have been injected between sedimentary strata belonging to the Jura-Triassic and Cretaceous systems. These masses, thirty-six in number, have consolidated in dome-shaped bodies, termed by Mr. Gilbert "laccolites," which have arched up the overlying strata, sending sheets, veins and dykes into them, and producing in them the phenomena of contact metamorphism. There is no proof that any of these protrusions communicated with the surface, and there is positive evidence that most if not all of them did not. The progress of denudation has laid bare the inner structure of this remarkable type of hill, and yet has left records of every stage in its sculpture. In one place are seen only arching strata, the process of erosion not having yet cut down through the dome of stratified rocks into the trachyte that was the cause of their uprise. In another place, a few dykes pierce the arch; in a third, where a greater depth has been bared away, a network of dykes and sheets is revealed; in a fourth, the surface of the underlying "laccolite" is exposed; in a fifth, the laccolite, long uncovered, has been carved into picturesque contours by the weather, and its original form is more or less destroyed.[358]
[357] See the remarks and diagram, ante, p. 86.
[358] "Geology of the Henry Mountains," by Mr. G. K. Gilbert, U.S. Geographical and Geological Survey of the Rocky Mountain Region, 1877.
The gabbro "laccolites" of the West of Scotland belong to an older geological period than those of Utah, and have, therefore, been longer subject to the processes of denudation. They have been enormously eroded. The overlying cover of basalt has been stripped off from them, though from the escarpments beyond them it is not difficult in imagination to restore it. In Rum it has been so completely removed, that only a few fragments remain at some distance from the core of gabbro, which now stands isolated. In Ardnamurchan, and still more in Skye, the surrounding plateau of basalt remains in contact with the gabbro bosses. But in Mull, where the plateau-basalts reach now, and perhaps attained originally a greater thickness than anywhere else, they have protected the intrusive sheets, which are therefore less deeply cut away than in any of the other districts, and no great central core of gabbro has yet been uncovered.
CHAPTER XLV
THE ACID ROCKS
Their Petrography—Their Stratigraphical Position and its Analogies in Central France
We now come to the examination of another distinct phase of volcanic action during Tertiary time in Britain. The igneous rocks that have been under consideration in the foregoing chapters, whether poured out at the surface or injected below ground, have been chiefly of basic, partly indeed, like the peridotites, of ultra-basic character. Some, however, have shown an andesitic or intermediate composition. Reference has also been made to the probable eruption of acid rhyolites in the long interval between the outflow of the lower and the upper basalts in Antrim. But we now encounter a great series, decidedly acid in composition, in the more largely crystalline members of which the excess of silica is visible to the eye in the form of free quartz. While there is a strong contrast in chemical composition between this series and the rocks hitherto under discussion, there are also marked differences in structure and mode of occurrence. Like the gabbros, all the masses of acid rock now visible appear to be intrusive. They have been injected beneath the surface, and therefore record for us subterranean rather than superficial manifestations of volcanic action.
The existence of rocks of this class in the midst of the basic masses has long been recognized. They were noticed by Jameson, who described the hills between Loch Sligachan and Broadford as composed of "a compound of felspar and quartz, or what may be called a granitel, with occasional veins of pitchstone."[359] Macculloch gave a fuller account of the same region, and classed the rocks as chiefly "syenite" and "porphyry."[360] In Antrim, also, even in the midst of the basalt-tableland, masses of "pitchstone-porphyry "pearlstone-porphyry," "clay-porphyry," and "greystone" were observed and described.[361] In more recent years Professor Zirkel has given a brief account of the so-called "syenite and porphyry" of Mull and Skye,[362] and the late Professor Von Lasaulx fully described the "trachyte" or rhyolite of Antrim.[363]