Another fact which may be regarded as now well established is the persistence of composition and structure in the lavas of all ages. Notwithstanding the oft-repeated cycles in the character of the magma, the materials erupted to the surface, whether acid or basic, have retained with wonderful uniformity the same fundamental characteristics. No part of the contribution of British geology to the elucidation of the history of volcanic action is of more importance than the evidence which it furnishes for this persistent sameness of the subterranean magma. An artificial line has sometimes been drawn between the volcanic products of Tertiary time and those of earlier ages. But a careful study of the eruptive rocks of Britain shows that no such line of division is based upon any fundamental differences.
The lavas of Palæozoic time have of course been far longer exposed to alterations of every kind than those of the Tertiary periods, and certain superficial distinctions may be made between them. But when these accidental differences are eliminated, we find that the oldest known lavas exhibit the same types of structure and composition that are familiar in those of Tertiary and recent volcanoes. Many illustrations of this statement will be furnished in later chapters. As a particularly striking instance, I may cite here the most ancient and most modern lavas of the Grand Cañon of the Colorado. Mr. Walcott and Mr. Iddings have shown that in the Lower Cambrian, or possibly pre-Cambrian, formations of that great gorge, certain basic lavas were contemporaneously interstratified, which, in spite of their vast antiquity, are only slightly different from the modern basalts that have been poured over the surrounding plateau.[16]
[16] 14th Annual Report U.S. Geol. Survey (1892-93).
The chief lavas found in Britain.—Of the lavas which have been poured out at the surface within the region of the British Isles, the following varieties are of most frequent occurrence. In the acid series are Rhyolites and Felsites, but the vitreous forms are probably all intrusive. The intermediate series is represented by Trachytes and Andesites (Porphyrites), which range from a glassy to a holocrystalline structure. The basic series includes Dolerites, Diabases, Basalts, Limburgites (or Magma-basalts, containing little or no felspar), and Picrites or other varieties of Peridotites. The intrusive rocks display a greater variety of composition and structure.
ii. VOLCANIC AGGLOMERATES, BRECCIAS AND TUFFS
The coarser fragmentary materials thrown from volcanic vents are known as Agglomerates where they show no definite arrangement, and especially where they actually fill up the old funnels of discharge. When they have accumulated in sheets or strata of angular detritus outside an active vent they are termed Breccias, or if their component stones have been water-worn, Conglomerates. The finer ejected materials may be comprehended under the general name of Tuffs.
Although these various forms of pyroclastic detritus consist as a rule of thoroughly volcanic material, they may include fragments of non-volcanic rocks. This is especially the case among those which represent the earliest explosions of a volcano. The first efforts to establish an eruptive vent lead to the shattering of the terrestrial crust, and the consequent discharge of abundant debris of that crust. The breccias or agglomerates thus produced may contain, indeed, little or no truly volcanic material, but may be made up of fragments of granite, gneiss, sandstone, limestone, shale, or whatever may happen to be the rocks through which the eruptive orifice has been drilled. If the first explosions exhausted the energy of the vent, it may happen that the only discharges from it consisted merely of non-volcanic debris. Examples of this kind have been cited from various old volcanic districts. A striking case occurs at Sepulchre Mountain in the Yellowstone Park, where the lower breccias, the product of the earliest explosions of the Electric Peak volcano, and attaining a thickness of 500 feet, are full of pieces of the Archæan rocks which underlie the younger formations of that district.[17] These non-volcanic stones do not occur among the breccias higher up. Obviously, however, though most abundant at first, pieces of the underlying rocks may reappear in subsequent discharges, wherever by the energy of explosion, fragments are broken from the walls of a volcanic chimney and hurled out of the crater. Illustrations of these features will be given in the account of the British Carboniferous, Permian and Tertiary volcanic rocks.
[17] Prof. J. P. Iddings, 12th Ann. Rep. U.S. Geol. Survey (1890-91), p. 634.
It will be obvious that where the component materials of such fragmentary accumulations consist entirely of non-volcanic rocks, great caution must be exercised in attributing them to volcanic agency. Two sources of error in such cases may here be pointed out. In the first place, where angular detritus has been precipitated into still water, as, for instance, from a crag or rocky declivity into a lake, a very coarse and tumultuous kind of breccia may be formed. It is conceivable that, in course of time, such a breccia may be buried under ordinary sediments, and may thereby be preserved, while all trace of its parent precipice may have disappeared. The breccia might resemble some true volcanic agglomerates, but the resemblance would be entirely deceptive.
In the second place, notice must be taken of the frequent results of movements within the terrestrial crust, whereby rocks have not only been ruptured but, as already pointed out, have been crushed into fragments. In this way, important masses of breccia or conglomerate have been formed, sometimes running for a number of miles and attaining a breadth of several hundred feet. The stones, often in huge blocks, have been derived from the surrounding rocks, and while sometimes angular, are sometimes well-rounded. They are imbedded in a finer matrix of the same material, and may be scattered promiscuously through the mass, in such a way as to present the closest resemblance to true volcanic breccia. Where the crushed material has included ancient igneous rocks it might deceive even an experienced geologist. Indeed, some rocks which have been mapped and described as volcanic tuffs or agglomerates are now known to be only examples of "crush-conglomerates."[18]