: Jean-Jacques Rousseau by Lema Tre Jules - Rousseau Jean-Jacques 1712-1778 FR Littérature

@FreeBooks

Posted in: #FreeBooks #FreeBooksIndia #FreeBooksApp #gg

Vote Up Vote Down   0 Reactions

Login to give Reaction!

FISSURE ERUPTIONS.

Those who have been accustomed to regard volcanoes such as Vesuvius or Etna as the one form of volcanic action, may be somewhat startled by the statement that lavas have sometimes been poured forth from fissures hundreds of miles in length, and have deluged vast tracts of country beneath sheets of molten rock, compared with which the puny lava-fields of Italy sink into insignificance. History, romance, and legend have been so long associated with the group of volcanoes overlooking the quiet Tyrrhenian Sea, that from the time when Pindar sung of the fire-floods of Etna, and Pliny died, too rashly investigating the great eruption of Vesuvius, till Scrope, Lyell, Von Buch, and Palmieri made them the centre of their researches, they have occupied too large a share of attention, and have been thus regarded as the full normal development of that volcanic activity of which they are but a phase, and only a minor phase. Hence, when, eighteen years ago, Richthofen described the great lava-plains of Western America, and attributed their origin to ejection from fissures, and not from vents, so firm a hold had been taken of the minds of geologists by nearly twenty centuries of observation of Vesuvius and its fellows, that his arguments were received with incredulity; and though they have been amply verified by subsequent investigations, and have afforded the clue to the interpretation of the vast series of volcanic rocks in other quarters of the globe, they have not been generally circulated, and few, outside the circle of geologists, are acquainted with them.

In this paper, we propose briefly to describe some of the most noted of these 'fissure'--or as Richthofen called them--'massive' eruptions, selecting as types that on the Snake River in the United States, and those in India, Abyssinia, and the north-west of Europe; and finally, to glance at their possible connection with the form of volcanic excitement more frequently displayed.

The one which has attracted most attention is that which formed the plateau of the Snake River, and which covers altogether, in Idaho, Oregon, and Washington, an extent of country equal to France and Great Britain combined. The district is one wide verdureless waste of black basalt, stretching westwards from the mountains by which it is bounded on the north and east, as an apparently boundless desert, black and bare, as though it had but recently cooled. Except for the shifting sand-dunes and slight ridges of basalt extending like long low waves or ground swells--to which Professor Geikie compares them--the surface would be quite level, the lava having either been poured over a plain, or having buried the undulations of the country beneath hundreds of feet of stone. The columnar structure, so often adopted by basalt, is here largely developed, and to it is due the tesselated appearance of the rock, which adds another to the many striking features in the scenery of the district. The only river in the district is the Snake River, which winds its way to the Pacific through a ca?on seven hundred feet deep, and which is joined, through underground courses, by the few streams that flow on to the basalt from the neighbouring mountains, and soon sink beneath the surface.

The Director-general of the Geological Survey visited the district five years ago, and his graceful pen has thus described his first view of this great lava desert: 'We had been riding for two days over fields of basalt level as lake bottoms among the valleys, and on the morning of the last day we emerged from the mountains upon the great sea of black lava which seems to stretch illimitably westwards. With minds keenly excited by the incidents of the journey, we rode for hours by the side of that apparently boundless plain. Here and there, a trachytic spur projected from the hills, succeeded now and then by a valley, up which the black flood of lava would stretch away into the high grounds. It was as if the great plain had been filled with molten rock, which had kept its level, and wound in and out along the bays and promontories of the mountain slopes, as a sheet of water would have done.'

The feature, however, that most struck Professor Geikie, as it had done previous observers, was the absence of ash and scoria, and of any crater where the eruption could have occurred. There are indeed a few cinder cones, but they are analogous--as he says--to the smaller cones on the flanks of a volcano, or more so to those around the vapour-vents on the surface of lava-streams. Such vast masses of lava were certainly not ejected from these, nor in the ordinary method of volcanic emission. We are therefore forced to accept Richthofen's theory, that they are due to a series of eruptions from fissures which stretched across the country for several hundreds of miles, but are now hidden by the sheets of stone in which, since no very remote period, the district has been enwrapped. Geologically speaking, this must have been recent, as is evidenced by the lava-floods having covered the present valleys, and having sealed up the gravel and silt of their lakes and rivers; but sufficient time has elapsed to have allowed of the erosion of the picturesque ravine of the Snake River; and in so dry a climate and on so hard a rock, this must have been slow work, though in all probability it does not carry back the date of the event beyond the human occupation of the continent. It is by no means impossible that man may have witnessed the last of these eruptions from the summit of the trachyte hills at the base of which was eddying this ocean of molten rock.

Another series of volcanic rocks that has long been a source of perplexity to geologists is that which, covering two hundred thousand square miles of India, is known as the Deccan Traps. Though the separate lava-flows are of no great thickness, they attain a total of six thousand feet, exclusive of the 'intertrappean' fresh-water deposits with which they are associated. The rock is mainly dolerite or basalt, but is very variable, and in many places it exhibits spheroidal or columnar structure; unlike, however, that of the Snake River, volcanic ash is common. The plateau formed by these deposits consists of a vast undulating plain, and of flat-topped hills with occasional 'scarps' or cliffs, which in the Sahyadri range are four thousand feet high, the whole being marked by terraces along the outcrop of the horizontal layers of basalt. In many points, the scenery of this district is much like that of the Snake River; but, owing to the greater age of the beds--belonging to the Cretaceous or chalk group--they are more weathered, and covered by a thin soil formed by the disintegration of the rock beneath, bear a slight vegetation. This, however, heightens the monotony, as it consists of a simple covering of straw-coloured grass; though, from March, when the grass is burnt, till the commencement of the rainy season in June, the black soil, the black rocks, and the black ashes of the vegetation, combine to produce a scene of the most solemn desolation. The scene can be well viewed from the railway between Bombay and Nagpur, which traverses this plateau for five hundred and nineteen miles without once leaving the lava.

Many ingenious theories have been started to explain the origin of these lava-fields. Some writers, as Newbold, hold that the beds were ejected from submarine volcanoes; but this is conclusively disproved, since no marine fossils are associated with them, and as the minute dust--due to the shattering of the ash and ejected masses by the sudden cooling--so characteristic a feature in subaqueous eruptions, is wholly absent. According to another school, of which Hislop and Carter were the leaders, the lavas were poured over the bottom of an enormous lake, in places 'so shallow as to allow the igneous rock to rise above its surface into the atmosphere,' thus giving rise to beds of ash; but as this assumes the existence of a vast fresh-water lake hundreds of miles long and broad and shallow throughout, for which no evidence has been adduced, this theory is discredited. One of the latest writers upon the subject--Mr W. T. Blanford of the Indian Geological Survey--rejecting the former hypotheses, argues for the former existence of volcanic foci in Cutch, in the lower Narbada valley and near the Sahyadri range, to the east and north-east of Bombay, though he admits that if his theory be true, the lava must have flowed for immense distances, and hence postulates its excessive fluidity. The possibility, however, of the rock having done so on a surface quite horizontal, and in the semi-fluid viscous condition in which most basic lavas are erupted, presents insuperable difficulties, and there is now hardly any doubt that these Deccan Traps were ejected in the same manner as were those of the Snake River.

For our knowledge of the series of volcanic rocks that covers the greatest part of Abyssinia, we are also largely indebted to Mr Blanford, who explored the district during the expedition of 1867. These rocks, widespread though they be, are but the remnants, as are also those of Arabia, of a fissure eruption that inundated Abyssinia and Southern Arabia to a depth of two or three thousand feet.

Nearer home, in North-western Europe, are the relics of the same form of volcanic activity, as evidenced by those disconnected patches of lava-streams and trap dikes, which, scattered over the north of Ireland, the Highlands of Scotland, and the northern counties of England, form such prominent features in the landscapes of those districts, as in the columnar basalts of Fingal's Cave and the Giants' Causeway, and the igneous dikes that cross, like walls of rock, the hills of our northern counties. The area that this eruption covered was at least one hundred thousand square miles; while, as it probably extended to the Faroes and Iceland, it may have been much larger. We are not left, as in the case of the Snake River, without evidence as to whence this mass of rock has come, for, since the Miocene Age, when it was ejected, denudation has attacked the district, and sea, stream, and ice have carried off most of the three thousand feet of lava that then covered the land, and have left but a few scattered fragments on which to reconstruct the record of the event. In doing so, they have bared the roots of the old fissures whence this mighty flood must once have welled; and we thus learn that we must trace its source to the long dikes that now stretch over the district, crossing from formation to formation, and traversing dislocations of thousands of feet without any break or change. Such dikes can be traced from end to end of the region, from Donegal to Fife, and from Yorkshire to the Faroes, increasing in numbers as we approach the volcanic regions of Antrim and the Hebrides. They did not all reach and overflow the surface, as is conclusively proved by Scotch mining operations, and by the fact that they sometimes disappear, to rise again elsewhere on the same line. Such may have been the case with all those of Yorkshire, as the evidence by which we might decide has all been swept away. Nevertheless, we know that a vast district was covered by the great fire-flood which was poured over the tropical forest that then flourished on the site of the Scottish Highlands.

From this brief description of the most important of these old fissure eruptions, we see that there is another and a grander phase of vulcanism than that now displayed either by Vesuvius or Hecla. This is unquestioned, and the sphere of speculation is removed to the relation between the two classes. It is to Richthofen that we owe the most plausible theory: he considers these massive eruptions as the fundamental development, and 'modern volcanic cones as merely parasitic excrescences on the subterranean lava reservoirs, very much in the relation of minor cinder cones to their parent volcano.' Thus the form regarded till recently as the one method of volcanic ejection, appears to be of but secondary importance, being merely a safety-valve to relieve the pressure from the lava-sources below; or may represent but a feebler and waning conn jardin fort ?lev? dans lequel la maison s'enfon?ait sur le derri?re, venaient ombrager la fen?tre et passaient quelquefois jusqu'en dedans. Je sais bien que le lecteur n'a pas grand besoin de savoir tout cela, mais j'ai besoin de le lui dire... .

? individualisme! ? romantisme!

Free books android app tbrJar TBR JAR Read Free books online gutenberg

(0)

Login to post a comment!