Page images

appearing in the Carboniferous, some in the JuraTrias, and others in the Cretaceous. From the lowest to the highest laccolith the range is not less than 4000 feet, those which are above not infrequentlyoverlapping those which lie below. "Their horizontal distribution is as irregular as the arrangement of volcanic vents. They occur in clusters, and each cluster is marked by a mountain. In Mount Ellen there are perhaps thirty laccolites; in Mount Holmes there are two ; and in Mount Ellsworth one. Mount Pennell and Mount Hillers have each one large and several small ones." The highest of these mountains attains an elevation of over 11,000 feet, rising some 5000 feet above the plateau at its base. The strata of which that plateau is built up are approximately horizontal, and appear at one time to have been covered by some thousands of feet of Tertiary deposits, the nearest remains of which occur at a distance of thirty miles from the Henry Mountains. Mr. Gilbert is of opinion that the laccolites were most probably intruded after the deposition of the Tertiary strata, and before their subsequent removal by erosion.

The whole structure of the Henry Mountains shows that the actual surface was affected by those intrusions, the horizontal strata being arched upwards so as to form dome-shaped elevations, rising prominently above the general level of the plateau. The laccoliths are all of considerable size, the smallest measuring more than half a mile, and the largest about four miles in diameter. The mountains formed by them consist of a group of five individuals separated by low passes, but having no definite range or trend. The subsequent erosion of these mountains, Mr. Gilbert remarks, has given the utmost variety of exposure to the laccoliths. In some places these are not yet uncovered, and we see only the arching strata which overlie them, the strata being cut across by only a few dikes or traversed by a network of dikes and sheets. In other places denudation has partly bared the laccoliths or even completely exposed them, so that their original form can be seen. In yet other places the bared laccolith itself has been attacked by the elements, and its original form more or less changed. It is even quite possible that occasionally laccoliths may have been entirely demolished, and that some of the truncated dikes now visible at the surface may mark the old fissures or conduits through which such vanished laccoliths were injected.

From the evidence just referred to, it is obvious that intrusions of igneous rock, if of sufficient thickness, are capable of warping the surface, and of forming more or less considerable elevations. But as erosion tends to reduce all such upheavals more or less rapidly, it is only those of relatively recent age that can retain any trace of their original configuration. All masses of intrusive rock of great geological antiquity, which now form hills and mountains, do so in virtue of their greater resistance to the action of epigene agents. They may have arched up the rocks underneath which they formerly lay buried, and so

[ocr errors]

produced more or less prominent elevations at the surface, but such primeval land-forms have been entirely removed—the features now visible are the direct result of erosion and denudation.

Of true volcanic rocks it is not necessary to say much. Their eruption at and near the surface gives rise to hills and mountains of accumulation, the general aspect and structure of which are sufficiently familiar. The typical volcano is a truncated cone, built up usually of successive lava-flows and sheets of loose ejecta. At the summit is the central cup, or crater, marking the site of the vertical funnel, or throat, through which the various volcanic products find passage to the surface. These are naturally arranged round the focus of eruption in a series of irregular sheets, beds, and heaps, which dip outwards in all directions. It is this disposition of the materials which gives its characteristic form to a volcano. The upper part of the cone inclines at an angle of 300 to 350, but this steep slope gradually decreases until towards the base the inclination may not exceed 3° or 50. In a typical volcano, therefore, the internal geological structure and the external configuration coincide—the mountain with its graceful outline is the direct result of subterranean action. It is obvious, however, that the quaquaversal arrangement of the lavas and tuffs is a weak structure. Many cones, it is true, are braced and strengthened by dikes and other protrusions of molten rock, which consolidate in the cracks and fissures that often traverse a volcanic mountain in all directions. But, although such intrusions may delay, they cannot prevent the ultimate degradation of a volcano which has ceased to be active.

Active and dormant or recently extinct volcanoes differ in form, to some extent, according to the prevalent character of their constituent rocks, and the manner in which these have been heaped up. Some cones consist of cinders, or other fragmental ejecta, with which no lava may be associated. Not infrequently, again, such cones have given vent to one or more lava-flows. From small cinder-cones, showing a single coutte, to great volcanoes built up of a multitudinous succession of lavas and sheets of fragmental materials, there are all gradations. The smaller cones are often the products of a single eruption ; while the larger cones owe their origin to many successive eruptions, between some of which there may have been prolonged periods of apparently complete repose. The beautiful symmetry of the typical cone is often disturbed. This is due sometimes to the shifting of the central focus of eruption; sometimes to the escape of lava and ejecta from lateral fissures opening on the slopes of the mountain. Not infrequently, also, the symmetry of a growing cone is liable to modification by the action of the prevalent wind, the loose ejecta during an eruption falling in greatest bulk to leeward.

Tuff-cones and cinder-cones range in importance from mere inconsiderable hills to mountains approaching or exceeding 1000 feet in height In the typical cinder-cone the crater is small in proportion to the size of the volcano; it is simply an inconsiderable depression at the summit of the cone. Occasionally, however, we meet with large crateral hollows, mostly now occupied by lakes ringed round by merely an insignificant ridge of fragmental materials. Sometimes, indeed, such large hollows show no enveloping ring whatsoever. Extensive craters of this kind are believed to be the result of explosive eruptions, and it is quite possible, or even probable, that their width has been considerably increased by subsequent caving in of the ground. Cinder-cones and tuff-cones vary in form according to the character of their constituent materials. When coarse slags and scoriae or pumice predominate, the sides of the cone may have an inclination of 350, or even of 400. When the materials are not quite so coarse, the angle of slope is not so great; it diminishes, in short, as the ejecta become more finely divided, so as sometimes not to exceed 15°.

Just as there are cones composed chiefly or exclusively of fragmental materials, so there are volcanoes built up of one or of many successive lava-flows, with which loose ejecta may be very sparingly associated, or even sometimes absent altogether. Lava-cones likewise vary in shape and size according to the nature of their component rocks. Some form abrupt hills of no great height; while others are depressed cones, attaining a great elevation and sloping at a

« PreviousContinue »