Rensselaer Union, Volume 8, Number 11, Rensselaer, Jasper County, 2 December 1875 — The Origin of Mountains. [ARTICLE]

The Origin of Mountains.

Mountains have been explained by two widely-different suppositions. One is that they are due to sediments deposited under water from the erosion of a wasting continent which by upheaval have become mountains. The other is that they are due to uplifts, as the result of lateral Sressure caused by shrinkage of the earth’s xterior. For the last fifteen years or 4 more these conflicting views have each been held by geologists of undisputed authority; ana “ when doctors disagree who shall decide?” is the old question which remains still unanswered. This uncertainty in all moral reasoning, when we have to balance probabilities, is a great source of discomfort to the youthful student; and perhaps in no department of human inquiry is this more true than in the field of science, for what is highly probable to-day may be shown in the light of advancing science to be highly improbable to-morrow. And in reply to the oft-repeated question of the young student: “ What is the use of learning as truth today what may be rejected as error tomorrow?” it may be said that all the successive theories of advancing science are stepping-stones which may eventually lead to the undoubted truth. We see use in taking the first faltering and ill-advised steps in any avocation, though we soon rejects these for others more conducive to the end desired. Without attempting a decision, we propose in this article only to state some of the main points in the arguments pro and eon, and leave all to decide for themselves as to which is the more reasonable. We would naturally conclude that, if mountains are due to lateral pressure, they would be formed by the uplifts or elevations of the earth’s crust. But this is seldom the fact, for several reasons. According to Prof. Dana, many if not all the mountains have their origin in the bending down of the crust. As the crust subsided, the trough was kept full of water, which continually deposited sediment. This deposition about kept pace with the rate of subsidence. In this manner many of our mountain ranges were, in the earlier ages, taking the initiatory steps in the process of mountain-making. As the crust subsided and was covered to a great depth with an accumulation of eroded material, it would be weakened by the earth’s internal heat. An addition of several thousand feet of sediment to the surface would bring a given degree of heat so many thousand feet nearer the surface. This would often be sufficient to soften or melt the sustaining crust, which would then yield before the lateral and vertical pressure combined and Cause the crusts on the sides of the trough to fold over and approach each other above it, thus crushing the sedimentary beds into a narrower space, with the necessary result of elevating the crushed and folded strata in the middle.

The Appalachian chain illustrates the that one mountain system may be formed by several successive depressions, accompanied with the deposition of eroded material. Prof. Hall attributes the cause of mountain-making to sedimentary accumulations, which, by their weight, are sufficient to cause a depression in the crust. Thus, by the addition of 40,000 feet of sediment the crust would sink the same number of feet. Then, by a subsequent elevation of the crust, the accumulated strata would be raised into a mountain, independent of lateral pressure. He just reverses the idea of Dana, by making the subsidence a consequence of sedimentary accumulations, instead of the accumulations a consequence of subsidence. To this Dana objects, because “ the earth’s crust would have to yield like a film of rubber to have sunk a foot for every added foot of accumulation over its surface, and mountains would have had no standing place.” Another reason why the elevations due to lateral pressure do not produce the high mountains appears in the fact that, when a senes of strata is sharply bent upward —forming an anticlinal—the outer strata are fractured and strained apart, while the strata which are bent downward—forming a synclinal—present to the surface a firm and compact mass. This can be clearly shown by making a sudden bend in a walking-stick. The fibers of the outer curve will oe torn asunder, leaving a splintered and ragged surface, while on the inner curve they will become unusually dense and firm. The fractured edges of the anticlinal curve are in a favorable condition to be worn away by water, while the compact surface of the synclinals, though forming the valleys where the greatest amount of running water would act upon it, suffers but little erosion. The consequence is that the elevated strata are worn away even below tliC level of the original valleys, and the latter become the elevations. This can be proyed by noticing that the strata visible ori the sides of most valleys and hills are not parallel to the sides, but are nearly at right angles to them. The mountains 'formed by depressions of the crust were far more common in the early history of sedimentary deposit, for the crust w‘aa thi-ii comparatively thin, and hence more yielding to lateral pressure. But after the crust became thickened beneath by the cooling of the earth, and more rigid by the accumulation of strata above and by previous plication and solidification, the mountains formed were largely due to uplifts of very wide extent carrying the stratified deposits with them. Our Rocky Mountain system was formed by these uplifts in the tertiary age, and it is probable that coral island subsidences in the Pacific Ocean accompanied the continental elevations. .The adherents to the accumulation theory—among whom are Hall and Hunt on this continent, and Scrope and Lyell in Europe—have noticed that in mountainous districts the elevations are less than the aggregate thickness of the strata, while in non-mountainous sections the heightscorrespond to the thickness of the strata. If the latter were equally true in mountainous districts, the Appalachian Mountains would aitain a height of 40,OOOTeet. Mr. Hall holds that these barriers are due to original deposition of riiaterials and not to any subsequent forces breaking up or disturbing the strata of wnich it is composed; and that upheavals and contortions of strata are only accidental and | local. In this view he is sustained by ‘

Montlosier and Jukes. He also claims that the direction of mountain elevations is determined by accumulations along thw sides of oceanic currents or shore lines. Dana, on the other hand, considers the northeast and northwest trends of most of the mountain and shore lines on the globe to be the result of cleavage in the earth’s crust, and to indicate lines of weakest cohesion, like cleavage planes in crystals. The accumulation theory supposes that after a vast amount of material has been deposited in successive strata under water a great continental upheaval brings the whole mass high and dry above the water line; and the present mountains are the stratified deposits which have escaped denudation by the action of frosts and floods. We have good illustrations ot this process of erosion in the Missouri River Valley, where the elevated land is being constantly washed away, forming deep ravines and abrupt ridges, and is carried into the muddy Missouri and deposited in the deltas at the mouth of the Mississippi, thus adding constantly to the territory of Louisiana. As Egypt is said to be a gift of the Nile so Louisiana is a gift of the Missouri. The effects of erosion on a small wentft ranteww nn the sides of deep railroad cuts, where miniature mountains and valleys are formed by the washing of water as it runs down their slopes. ( Prof. Le Conte opines that these opposing theories result from the loose use of the word mountain. He treats the whole subject under the two heads of mountain formation and mountain sculpture, and claims that the true mountain chain or the convex plateau which constitutes it is due only to foldings of the crust and that those elevations which are left by the erosive action of water are not mountains but simply sculptured continental elevations. The effect of shrinkage and of erosion can be fairly seen, on a small scale, by the following artificial contrivance: Take a well-filled bladder, or toy rubber balloon, and cover it completely with several successive coatings of tallow, glue, plaster of Paris, or other substances that will harden after they have been put on in a plastic state. These will represent the stratified crust. Then, by withdrawing some of the air from the bladder, which will. aujjjßk to contraction of the nucleus, the crust will become rigid, furrowed and fractured by lateral pressure, like the crust of the earth. Now, by allowing a well-regulated stream of water to flow over the surface of this, we can see many of the phenomena of erosion, like those apparent on the earth’s surface.— Scientific American.