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and forced up above the general level, and thus form mountain chains. The first supposition is untenable, since the elevated portions of the crust would be brought to a lower level by the force of gravity alone; so that, if such a condition of things were possible, it could not long be maintained. According to the other supposition, the parts where the crushing together took place would gradually become thicker (for these changes could not be sudden in any ordinary sense of the word), and more and more unyielding, until an equilibrium was reached. We easily see that these changes would either produce plication of strata if plastic, or broken strata if rigid. We, therefore, find that the immediate cause of the elevation of mountain ranges is the folding and bending up of strata. The subjacent matter would effectually prevent any great amount of increase of thickness downward.

This hypothesis of the origin of mountain masses explains in a very satisfactory manner the origin of slaty cleavage. This cleavage exists in most mountains. According to the experiments already cited, where a great pressure was applied to plastic clay, a cube of the material would, on the average, according to Professor Le Conte, be crushed from two and a half miles into one, and, by supposing the whole extension in a vertical direction, 10,000 feet of thickness would be pressed into 25,000 feet, or an increase in thickness of 15,000 feet, which would produce a mountain of considerable elevation.

Professor Hall was the first to draw special attention to the fact that mountain chains are composed of immense masses of sediments, even much thicker than the mountains themselves. We hence see that what are mountain masses now were once the bottom of seas of greater or less extent, for the sediment must have been deposited at the bottom of bodies of water. Where the Appalachian chain now stands, there existed an ancient ocean, called by geologists the Paleozoic Ser. The sediment deposited in this sea, and which now forms the Appalachian range, was washed largely from a great continental mass lying to the north, called the Laurentian area. A discussion of the facts in relation to the mountains lying in the western part of this continent leads to similar conclu

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sions in regard to their origin. After the Rocky Mountains were formed, where the Sierras now exist a sea probably existed, and after the elevation of the latter range the seacoast was removed to the west considerabiy further, and succeeding this change, the coast range was formed, and the great Pacific Ocean is the sea which now washes the western margin of this contiuent.

Extensive chains of mountains, like the Rocky and the Andes, were, then, according to this view, raised up from the bottom of extensive seas; and we may conclude that smaller and irregular ranges are the sediment deposited in interior seas of less extent. The reader will now ask how it is that sea-bottoms are more likely to become the lines along which the surface yields to the horizontal thrust, as we have explained. This we shall explain in a few words. Mr. Babbage and Sir John Herschel first drew attention to the fact that the interior heat of the earth would extend itself into the sediment, which, as we have seen, accumulates to a great depth. Thus the influence of the water and the interior temperature would soften the ocean beds and cause them to yield to the pressure exerted by the contraction of the earth, and in due course of time the mountain would be produced. According to this view, the beds of the two great oceans may be now rising; and soundings in the Gulf stream lend some plausibility to this view.

Professor De Conte sums up in relation to this hypothesis as follows: “It explains satisfactorily the following facts. 1. The most usual position of mountain chains near continental coasts. 2. When there are several ranges belonging to one system, they have usually been formed successively coast-ward. 3. Mountain chains are masses of immensely thick sediments. 4. The strata of which mountains are composed are strongly folded, and where the materials are suitable, affected with slaty cleavage, both the folds of the cleavage planes being usually parallel to the mountain chain. 5. The strata of mountain chains are usually affected with metamorphism, which is great in proportion to the height of the mountains and the complexit: of the foldings. 6. Great fissure-eruptions and volcanoes are usually associated with mountain chains. 7. Many other phenomena—such as fissures, slips, earthquakes, and the subsidence preceding the elevation of mountains, it equally accounts for.” We inay further add that it accounts for the existence of marine shells on the tops of mountains.

Fissure-eruptions, to which reference has been made, are phenomena which afford good proof that igneous causes were once more active in modifying the present appearance of the earth than they are at present. The great amount of lava that exists in the Sierras and the coast ranges, and especially in the former, renders it impossible that it could have flowed from volcanic craters. In the middle of California immense separate streams of lava begin, but in the northern part these become a general food not less than two or three hundred feet thick, and this continues throngh Oregon where the flood of lava is still more extensive, and the thickness has increased to two thousand feet; thence on through Washington Territory and into British Columbia to an unknown distance, the whole embracing an seven or eight hundred miles long, and eighty to a hundred in width. The lava has completely covered this tract, and where the Columbia river cuts through it, it is from two thousand to three thousand feet thick. There are a score, perhaps, of extinct, volcanoes scattered over this area, but it seems impossible that the great amount of lava to which we have referred could have ever flowed from their craters.

Supposing the preceding theory to be approximately correct, it may not usually happen that inountains are still rising in any given period of time, though there seems to be some evidence that the Andes are still increasing in elevation above the level of the sea. It seems that they have been gradually rising century after century at the rate of several feet, while the pampas on the east have been raised only a few inches in the same time.* If there be liquids and gases beneath the earth's surface, the pressure exerted by the con

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* Lyell's Prin. of Geol., Vol 1., p. 129.

traction of the crust might cause them to change their position. If one part of the crust should sink (and this might take place if the equilibrium of the forces in action were disturbed), the fluid parts would be driven in greater quantities to other places, and this process would assist in lifting up other parts. That the molten material docs change its position is indicated by Vesuvius and such volcanoes as seem to have been extinct for many centuries, but afterwards become active. Extinct volcanoes, at present so called, also indicate the same thing, especially if volcanoes are as numerous now as in former periods.*

The unstableness of the apparently solid crust of our globe is shown by the fact now well settled, that parts of the Scandinavian peninsula are gradually rising, while other parts seem to be gradually sinking. The Swedish naturalist, Celsins, early in the eighteenth century, gave his opinion that the waters of the Baltic sea and the Northern ocean were gradually sinking. From numerous observations he inferred that the rate of depression was about thirty-nine inches in a century. Several objections were raised against these conclusions, and some of the proofs given by Celsius were immediately controverted by the philosophers of his day.

Since the time of Celsius, however, it has been conclusively proved that the relative height of the land and water is variable. In 1807, after an examination of the Scandinavian peninsula, M. Von Buch makes this statement: “It is certain that the level of the sea cannot subside; the equilibrium of the waters forbids it. Yet the phenomenon of their retreat is no less unquestionable, and there remains but one admissible idea —that of a general upheaval of the land from Frederickshall to Abo, and perhaps to St. Petersburg.” At the suggestion of Celsius, marks had been cnt in the rocks at Pefle and Calmar; and Linnæus had also traced a level on a block, which he describes with precision. In one place a maritime city bad

* That mountain chains have been the yielding line in the shrinking of the crust, because the solid parts were thinner, and more molten matter beneath, is rendered probable by the fact that mountains are often volcanoes, and many have been, but they are now extinct, the liquid having solidified. * Principles of Geology, Vol. ii., p. 188. | Id., pp. 195-6.

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become an inland one, and in another an arm of the sea had become transformed into a highway. All these facts led the people of the country to the belief that the waters were diminishing. The conclusion of Von Buch, which we have just given, was the first expressed by a practical geologist after a personal examination.

The observation of Lyell in 1834, and MM. Leren, Erdmann, and Nordenskiöld, since then lead to the same conclusion as that expressed by Von Buch. “ After a review of all that has been said and published on this subject since the commencement of this century,” says Lyell, “ I am inclined to believe, with the pilots, fishermen, and engineers, that a slow alteration in the relative level of land and sea is taking place along certain parts of the Swedish coast.' The elevatory force, however, has not been constant in its action, but intermittent, with long pauses of rest.

Since Scandinavia has been comparatively free from violent earthquakes within the times of authentic history, the rise of land there seems a strange phenomenon ; but in the southern part it seems to be subsiding. Observations in Greenland show that a portion of that country also is subsiding. The instances to which we have now referred are sufficient to show that these slow changes in the relative level of the land and water, if continued in the same direction, will in the course of time produce new continents and islands, and new ocean beds. Man, continually adapting himself to these changes, scarcely perceives that they are taking place. It is the sudden changes which sometimes affect large though limited areas, that more especially attract man's attention, and teach how unstable is the solid earth on which he lives. These changes are brought abont by earthquakes, which seem to affect, at one period of time or another, nearly, if not quite, every region of country on the globe. “ Earthquakes manifest themselves by quick and successive vertical, or horizontal, or rotatory vibrations. In the very considerable

# Id., pp. 196–7.

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