Page images

stage in the process of demolition. Isolated cones and groups of cones crumble away, until all the lavas and tuffs ejected from the old vents may have disappeared, and the only evidence of former volcanic action that may remain are the basal portions of the dikes that proceeded from the foci, and the solid cores with which the latter were finally plugged up. (See Fig. 75.) As these cores usually consist of more


Fig. 75. View Of Neces = Cores Of Old Volcanoes. (Powell.)

durable materials than the rocks they pierce, they tend to form somewhat abrupt conical hills. It goes without saying that such extreme cases of denudation are met with only in regions where volcanic action has for a long time been extinct. Excellent exam


pies on a relatively small scale are furnished by the so-called "Necks" of Scotland, of which the accompanying section (Fig. 76) shows the general phenomena. Similar structures occur in many parts of Europe and North America.



Fig. 76. Section Of Highly Denuded Volcano. Minto Hill,

A■, throat or neck of volcano plugged up with ejectamenta, angular and subangular stones, grit, dust, etc.; St Silurian rocks; D, Old Red Sandstone strata.

Frequently the products of great volcanic eruptions of vast geological antiquity have been largely preserved, owing to their subsequent burial under sedimentary accumulations. Many of the hill-ranges of Central Scotland, for example, are built up of lavas and tuffs. These are the relics of volcanoes which came into existence in Palaeozoic times, and after erupting molten and fragmental materials for longer or shorter periods, eventually died out, becoming submerged and covered with sedimentary accumulations to depths of several thousand feet. Subsequent elevation of the region brought these sediments under the operation of the agents of erosion, and in time great thicknesses were removed, so that ultimately the ancient volcanic rocks were again laid bare and in their turn exposed to denudation. But if the lat

[ocr errors]

ri f

ter now form hills, it is simply because they consist for the most part ot more durable rock than the formations amongst which they lieIt is needless to say that all trace of their original configuration has disappeared. Indeed that had already vanished before the extinct volcanoes became entombed. Now ±r.d again the sites of the old foci ot eruption seem to be indicated by bosses and dikes of intrusive rock. but the general form and aspect of the hills are solely the results of erosion, determined and guided balneological structure and the nature and character of the old volcanic materials. They are true hills of circumdenudation. (See Fig. 77.) The massive or fissure eruptions of former times have in like manner been largely modified by subsequent epigene action. Although some of these belong to a comparatively recent geological period, they have yet been so carved and cut up, that their original plateaucharacter has become obscured or even lost Yet there can be no doubt that they formerly existed

as broad plains and plateaux, occupying many thousands of square miles. The older hills of Iceland, all the Faroe Islands, and the basalt hills of the Inner Hebrides and Antrim are the relics of vast plateaux, which were all probably at one time connected. The general aspect of the hills carved out of such plateaux is well illustrated by the Faroe Islands, to which some reference has been made in Chapter III.

It is believed, as already mentioned, that massive eruptions have proceeded rather from systems of fissures than from separate and individual foci, after the manner of most modern volcanoes of the cone and crater type. During the eruption of the plateau-basalts of Antrim and the Inner Hebrides, molten rock underlay not only those regions, but wide areas beyond, in the north of Ireland and through out central and southern Scotland and the north of England. All these areas are traversed by dikes of basalt, which become more and more abundant as they are followed towards the regions occupied by the basalt-flows. It is from these dikes that the latter appear to have proceeded. From the dikes that are now seen striking across Scotland and the north of England probably no outflow of lava took place ; the fissures up through which the molten rock came did not in those regions reach the surface. They are now exposed simply owing to denudation. Not a few dikes indeed still lie concealed. In the coalfields these are found cutting across the lower seams, but wedging out before the upper seams are reached.


The larger dikes in central Scotland often form conspicuous objects in a landscape. Owing to the superior durability of the basalt, they rise above the surface of the sedimentary rocks they traverse, and may occasionally be followed for miles, running as they do like great walls or prominent ridges across dale and hill. As examples may be cited two large parallel dikes which may be traced for many miles from Friarton Hill, near Perth, in a westerly direction. Near Dupplin, the more northerly of the two gives rise to a long prominent bank, which is followed for some miles by an old Roman road. In the neighbourhood of Crieff both dikes are equally conspicuous, rising as bold wall-like ridges, the more prominent of the two forming the steep crag upon which Drummond Castle is perched. When dikes cut through rocks as durable as themselves they cease to produce any marked feature at the surface. On the other hand, when the rocks traversed by them are the most resistant, the presence of the dikes is indicated by long trenches or hollows at the surface. Nothing could be so impressive and suggestive of the potency of long-continued erosion than the cropping out of these remarkable dikes. Their intrusion appears to have taken place in Tertiary times, and the great majority of those which occur in the mainland of Britain never actually communicated with the surface at the time of their formation. They cooled and consolidated below ground, yet we now see them laid bare not only in the low grounds and

« PreviousContinue »