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in general a rapid increase of heat in descending; but the increase does not follow the same law in all parts of the earth; nor does there appear to be any law dependent on either the latitude or longitude. He thinks that we may take an increase of 1° Fah. for every 45 feet, as a general average that will be not far from the truth.*
An artesian well in Charleston, South Carolina, bored to the depth of 1,108 feet, gives an average increase of 1° Fah. for 52 feet of descent, but the heat does not increase regularly with the depth, but it was greatest about those places at which streams of water were encountered.
But we have already given a sufficient number of observations to show the general result to which they lead, namely, a gradual increase of temperature froin the surface of the earth towards the centre, to, the greatest depth yet attained. If we assume as the most probable average result that there is an increase of 1° Fah. for an increasing depth of every 60 feet, we shall not be far from the truth. This increase of temperature is sufficient, as we may easily see, to carry us, after a few miles of descent, to a point where all known rocks would be fused if the conditions were the same as at the surface; and this has led many geologists, heretofore, to the conclusion that, except a crust of quite limited thickness, the earth is in a molten condition. But the greater number of the substances which compose the earth's crust have their fusing-points raised, as we have already mentioned, by pressure; and since the pressure exerted at great depths must be enormous, and it increases with the depth, we see that the melting-point will descend lower and lower, and at what tenperature that point will be reached we cannot at present tell.
The hypothesis of a molten nucleus of great extent led geologists very legitimately to the conclusion that igneous causes of change were formerly much more powerfuland active than at present. If we extend our period bick sufficiently we shall most certainly arrive at such a time; but that any effects of such action have left their impress on existing strata may
well be questioned; that is, to the extent that has generally been supposed.
It seems difficult for scientific men to rid themselves of the idea that nature is limited in time in performing her operations. Thus it was that the earlier geologists imagined changes in the earth's crust to take place with violence, and in short periods of time, since it was thought necessary to limit the time during which the earth's crust was assuming its present condition to a comparatively short period. So much for the influence which a myth has exerted on the progress of science. The advance of physical science is measureably in proportion to the liberation of the human mind from the bondage of authority.*
Dr. James Hutton, towards the close of the last century, introduced quite a new order of investigations, by endeavoring to show that existing causes are sufficient as far as we can legitimately look, when we would account for the existing order and arrangement observable in the structure of the crust of the earth. Playfair still further illustrated and explained, in elegantly written works, the ideas of Ilutton. In still more recent times Lyell has, by his patient investigations, varied acquirements, and learned works on the subject, done much to show that causes now in operation are wellnigh sufficient to produce all the phenomena observable in the mineralogical structure of the earth. Still we must not forget to call the attention of the reader to the factexemplified in many ways—that he who attempts to push a theory to its utmost extent is in danger of not seeing all facts in their true light. We are too apt to jump from one extreme to another, leaving some one else, who can philosophically view both sides, to draw the legitimate conclusion from the whole range of existing facts. It is difficult to take into consideration all krown causes, powerful and weak, and allow
* The authority of Newton in retarding the progress of physical optics has been referred to by Prof. Tyndall in his recent lectures in New York ; but the influence of theologic cosmogony in retarding the progress of geologic science by preventing the otherwise philosophic mind from making the best use of the facts of nature, has been still more marked.
for them to the extent, neither more nor less, which the nature of the case requires. . Although the influence of causes now in operation is probably sufficient to produce, the time being unlimited, by far the greater part of the changes which have taken place in past ages, in the earth's crust, yet it seems to be highly probabla that igneous causes, at least, were, in the early periods of the earth's physical history, rather more powerful and active than in more modern periods. Some proof of this will be given in its proper place.*
The principal inequalities of the earth's surface have been produced by the action of igneous causes acting from beneath the surface. Water is, and it has been, a powerful agent in changing the general features which we observe, but it is a great leveller, and never, with some insignificant exceptions, produces superficial inequalities. Water loosens the materials on which it acts, and thus enables the force of gravity to bring then to lower levels, and in this way produce a general equilibrium of surface. Igneous forces, on the other hand, in general tend to produce inequalities of the surface by elevating, which may cause depressions in others. Three-quarters of a century ago, geologists had but little correct knowledge respecting the structure of mountains and mountain chains, and of the causes which elevated them. Pallas and De Saussure, near the close of the last century, made some useful observations on the general structure of mountains; but it was not till the time of Von Buch, and after him M. Elis de Beaumont, that much progress was made in the investigation of the anatomical structure of mountains, if the expression may be allowed. According to the observations of Beaumont, the older strata, or those which are in general found lowest in the series, lying next to the granite core of the mountain, are inclined as if they had been raised up when the mountain was elevated; while resting on these are others and newer strata which have a horizontal position. Thus it would appear that by observing the kind of strata which are inclined, and those which are horizontal, we can determine the relative ages of mountains and mountain systems; for if some of the older series of strata were horizontal in one case, and inclined in another, our legitimate conclusion from these data would be that the latter mountains were elevated more recently than the former; while both might be older than other mountains which show an inclination of still newer strata. These were the principles which guided M. de Beaumont in his extensive generalizations on this subject.*
* The present physical constitution of the sun is a type on a grand scale of what the earth once was, if the nebular hypothesis has a foundation in nature. The solar spots are perhaps vast islands or continents, which are formed by the cooling of parts of the solar surface, and then by sinking they again become melted. The early islands and continents on the earth were probably subjected to such vicissitudes.
In some cases, and perhaps many, the strata are not only inclined, but they are folded and contorted as if they had been subjected to a powerful lateral pressure while they were in a plastic state. Whatever theory of mountain formation we adopt, it must account for this plication of strata. Our theories are often defective for the want of properly conducted experiments under conditions as nearly resembling those which nature presents as possible. The observations and experiments of Sharpe, Sorby, Houghton and Tyndall, show quite conclusively that slaty cleavage is due to pressure acting under particular circumstances, the resulting planes of cleavage being perpendicular to the direction of the force of pressure. Tyndall shows that a fine clay or almost any impalpable material, when subjected to pressure, and allowed at the same time to spread laterally, assumes a laminated structure. These facts are of high importance in explaining the formation and structure of monntain masses.
Observations on the strata surrounding mountain masses show, as has already been mentioned, that they are often much curved and distorted, as if they had become somewhat softened, and then subjected to great pressure. These distortions are especially found in the older strata. The continual
* Systèmes de Montagnes, 1852. naturelle,” tome xii.
d'Hist. denudation of exposed strata has changed their outward appearance very much. In many cases newer strata have been formed covering the inclined and curved ones, the former being horizontal. All hypotheses, the object of which is to explain the modus operandi of the formation of mountain systems, take into consideration this distortion of strata, but in nearly every one it is made a subordinate part of the theory. One recently proposed by Professor Joseph Le Conte makes this folding of strata the principal cause which immediately gives rise to the elevation of mountain chains. Since this hypothesis seems at present to be by far the most plausible of any yet imagined, we shall briefly explain its general features.
Suppose, then, the earth has reached, in the process of cooling, a condition some such as we have already explained, namely, a solid crust sufficiently stable to be no longer subject to such changes as would destroy any part of it, while beneath it the heat was sufficiently great to maintain some portions in a molten condition, or at least possibly so, the great nucleus beneath being perhaps mainly solid from a loss of temperature and from pressure. The liquid parts would be perhaps rather streams than a continuous stratum, extending completely around all parts of the earth. Surrounding space, notwithstanding the influence of the sun, would still be so much colder than the earth, that the refrigeration of it would yet continue, and the earth, as a whole, must grow smaller by contraction. The outer crust—whose thickness would necessarily be limited to a very few miles—would be so nearly cold with respect to surrounding space that the heat which it received from the interior would be radiated from its external surface without the temperature of the crust itself being much increased or diminished. Such a crust could not possibly be sufficiently self-supporting to be maintained as a shell, disconnected from the interior and much more heated nucleus. The result would be that the crust would give way in some parts by sinking down, and in others by rising up, thus changing the level surface into hollows and elevations, or, if this did not take place, the crust would be crushed together in the weaker parts,