Imágenes de páginas

that all the carbon was preserved. This is in the highest degree improbable, not to say impossible. Probably much more than half was returned to atmosphere in the form of carbonic acid and carburetted hydrogen. Again, we have taken no account of the enormous periods of time during which there was no carbon deposited on the spot in question, and represented by the intervening strata of limestone, sandstone, and shale. The estimate we have given above, therefore, probably falls very far short of the truth. Let us try another.

According to Messrs. Lyell and Dawson the coal strata of Nova Scotia are about three miles in thickness at the South Joggins. At another point, nearly 100 miles distant, (Albion mines,) they found the thickness nearly the same. There is little danger, therefore, of erring on the side of excess, if we take the average thickness of the strata over the whole basin at one and a half miles. Now, the area of this coal field, according to Mr. Lyell, is about 3,600 square miles. This would give, as the solid contents of these strata, 54,000 cubic miles. But we have already seen that this enormous amount of matter was almost certainly accumulated at the mouth of a great river. Let us see how long it would take one of our great rivers to do the work. I shall select for this purpose the Mississippi and the Ganges, because they are both very large rivers, carrying vast amounts of sediment, and because accurate observations have been made as to the amount of sediment brought down by them. These observations have been made upon the Mississippi by Drs. Forshay and Reddell, of New Orleans, and by Captain Strachey, British engineer, upon the Ganges. According to these observations it would take the Mississippi 2,000,000 years, and the Ganges* 375,000 years to perform the work. And yet the period we are now discussing is probably not one-thirtieth, certainly but a small portion of the entire geological history of the earth.

It will no doubt be objected to this estimate that it is founded upon a particular theory, and this theory may be incorrect, and the estimate thus falls to the ground. In answer to this objection it is only necessary to state that we are acquainted with no other circumstances under which strata accumulate so rapidly as at the mouths of rivers. Any other conceivable theory, therefore, would only increase the time.

Again, it will probably be objected that the agencies of nature may have been and probably were more active in earlier periods of the history of the earth than now. Such a notion, although almost universal among intelligent people and very prevalent even among geologists, is, as it seems to me, utterly without foundation in reason. In reference to this point geologists may be divided into two classes. The first and most numerous class hold that the agencies of nature have gradually decreased in activity from the earliest times until now. The other, to which Mr. Lyell and his followers belong, believes that.these agencies have acted much as they do now through all time; that there has been no progressive change of any kind, neither in the earth nor its inhabitants. Now, it seems to me that it can be proved, or at

This amazing difference in favor of a smaller river is due to the fact that the Ganges, being a tropical river, the rains all fall during six months, and are therefore very heavy. The washing of the soil and resulting sediment are necessarily in proportion. The mountainous country in which the Ganges takes its rise contributes also to the same result.

least rendered extremely probable, that neither of these theorists is in the right; that, in fact, while the igneous agencies have been decreasing in activity, the aqueous have been constantly increasing in the same proportion. As I believe I differ from all other geologists in my views on this point, I deem it important to go a little more fully into this subject.

It is generally admitted by geologists, and indeed there is good and substantial evidence of the fact, that the earth has been gradually cooling throughout all geological history from an original very high temperature. We have also, as I believe most geologists will admit, good and substantial evidence that the land has constantly increased both in extent and in elevation with the course of time, while the ocean has as constantly decreased in extent in the same proportion. In other words, these two elements, land and water, have been, as it were, gradually differentiated. Admit these two points and all the rest logically follow.

The activity of igneous agencies depends upon the internal temperature of the earth. As this has constantly decreased the igneous agencies have also decreased in energy in the same proportion. The aqueous agencies, on the other hand, are the result of currents of air and water upon the surface of the earth; and the rapidity of these currents depends, not upon the mean surface temperature, but upon the difference of temperature in different parts of the surface; i. e., between pole and equator or between land and water. It only remains to prove, then, that this difference of temperature has been constantly increasing with the course of time.

Land, as is well known, is both a better absorber and a better radiator of heat than water; i. e., will both heat faster and cool faster under given circumstances than water. A globe of land would be both hotter at the equator and colder at the poles than a globe covered with water and exposed to the same influences. Although the mean temperature would be nearly the same in the two cases, the difference of temperature would be much greater in the former than in the latter. It follows, therefore, that as the extent of land increased and that of the ocean decreased with the course of time the difference of tempersture between pole and equator must haveincreased in the same proportion. The gradual decrease of the mean temperature would evidently contribute to the same result; for it is evident that with a higher mean temperature a larger portion of water would exist in the form of vapor. This excessive vapor would rise into the atmosphere and become condensed into universal clouds, mist or fogs, but seldom, and to a very limited extent, in the earlier periods of the earth's history, into rain, because, as yet, there were neither extensive high land nor cool currents sufficient for extensive precipitation. Thus would result a thick, murky atmosphere, enveloping the whole earth. The necessary effect of this would be still further to prevent absorption of heat at the equator and radiation at the poles, and thus to produce still greater uniformity of climate. In the earliest geological periods, therefore, when the surface temperature, from internal causes, was very great, and the ocean almost universal, the difference of temperature between pole and equator was reduced to a minimum. In such a condition of things it is evident

that the exchange between pole and equator currents of the aqueous and aerial ocean must have been not only very sluggish but perfectly regular northeast and southwest currents in the northern hemisphere, and northwest and southeast currents in the southern. In proportion as the earth cooled the diversity of temperature between pole and equator became greater and the exchange more rapid. In the meantime the gradual increase in the extent and elevation of continents would introduce still greater diversity. The regular oceanic currents, by impinging upon the continents, are reflected in various directions, increasing still further the diversity of climate. Currents of the air, too, are no longer only trade winds, but also monsoons, land and sea breezes, &c. These various currents, mingling and contending, produce the infinitely varying winds of the present epoch. But the most important current we have not yet spoken of. Land and sea may be considered the two poles of a circulating apparatus; water rises in the form of vapor at one pole, passes over through the atmosphere, and is condensed on the other in the form of rain, and so back by the rivers to the ocean. The more rapid the condensation the more rapid the evaporation and the more rapid the circulation. Within certain limits, (i. e., until the land is sufficient to condense all the water evaporated from the ocean,) the amount of evaporation and condensation is in proportion to the extent and elevation of the continents. It is evident, then, that in the earlier periods of the earth's history, when the ocean was almost universal, although the air was saturated with moisture, there was comparatively little rain ; and that just in proportion as the continents increased in extent and elevation, evaporation, and condensation would increase in the same proportion. It is impossible to resist the conclusion, then, that from the earliest periods until now there has been a constant increase in activity and variety of currents of ocean and atmosphere; of wind and rain; of cloud and sunshine ; of fountains and rivers ; in fact of all that constitutes the life, variety, and beauty of our beloved earth.

Thus it appears that at first igneous predominated over aqueous agencies. It was this very predominance which caused uncompensated, progressive change-development of the earth as a whole; for perfect balance is incompatible with developement. But gradually aqueous agencies increased in energy; the antagonistic forces approached a balance as the earth approached maturity, until at present the balance may possibly be complete.

In all I have said I have had in view, of course, only the ordinary regular operation of aqueous agencies, or what Mr. Lyell calls “ now in operation.” I say of course, because the extraordinary, irregular operation of these agencies, such as are called debacles,' &c., are too uncertain and hypothetical, not to say improbable, to form the basis of any reasoning whatever. I repeat, then, that during the coal period the ordinary operation of aqueous or degrading agencies must have been more slow than at present. The accumulation of a certain amount of material in a river delta, other things being equal, would require a longer time than now.



It is probable, from what evidence we have on this subject, that the climate of the coal period was characterized by greater warmth, greater humidity, and greater uniformity than now obtains, and that the air was more highly charged with carbonic acid. Of the greater warmth of the climate we have evidence in the astonishing luxuriance and universal tropical character of the vegetation of the period. One of the most marked peculiarities of the flora of coal everywhere is the great relative abundance of ferns and fern allies. In the present flora of Great Britain the ratio of ferns to flowering plants is about 1 to 35, while in the coal flora of the same country nearly one-half of all the known plants are ferns. In the American coal fora the proportion of ferns is said to be still greater. That this abundance of ferns indicates a tropical climate is shown by the fact that in the existing flora, out of about 1,500 known species of ferns, 1,200 are confined to the tropics, and as we pass from the equator towards the poles the proportion of ferns, steadily diminishes. The same may be said with reference to the club-mosses. It is worthy of remark, too, that although conifers are abundant now all over the earth's surface, still those most nearly allied to the conifers of the coal—such, for instance, as the araucaria and salisburia of the present day—are found only in tropical regions. Now, during the coal period, this tropical vegetation extended as far as 75° north latitude. Tree ferns and gigantic club-mosses covered the spot now occupied by the Mellville island. The evidence of remarkable humidity is no less satisfactory, for it is only in warm, moist climates that ferns and club-mosses grow in the greatest abundance and luxuriance. On some islands in the tropics and in the south seas the abundance of ferns even approaches that of the coal flora. In fact, as a condition of the growth of these plants, moisture seems even more necessary than heat.

It has been objected to the greater heat of the climate, that coal was evidently formed by accumulation of carbonaceous matter in situ as now in peat bogs, and that peat bogs are found only in cool climates. The answer to this objection is not difficult. It is not the heat immediately, but the resulting capacity for moisture, or, in common language, dryness of the air of the tropics, which under ordinary circumstances prevents the preservation of carbon. The air is not so constantly at or near the point of saturation. Fogs, and mists, and clouds are not so constant as in cooler climates. But we have supposed greater humidity as well as heat during the coal period. Under these circumstances, there is no reason why peat should not accumulate. We see proof of this in the peat swamps at the mouth of the Mississippi. Here we find peat accumulating in great abundance in a climate which is yet very warm ; and we have already seen that it is in such peat swamps, rather than in the bogs of cooler climates, that we are to look for analogies with the peaty accumulations of the coal period. The enormous extent of these peat swamps becomes in its turn an additional proof of the great humidity of the climate.

The uniformity of climate—i. e. the comparatively equable distri

bution of heat and moisture on the surface of the earth during the coal period—is evidenced by the remarkable uniformity of the flora. The general character of the coal flora was very much the same in every portion of the earth's surface, and in many cases even the same species are found in the most distant countries. Thus many identical species have been found in Europe, United States, New Holland and Mellville island, countries the existing flora of which differ entirely. Now, although I cannot accede to the doctrine that diversity of climate is the physical cause of diversity of fauna and flora, yet, whether we consider the physical or the final cause, the result would evidently be the same, viz: the perfect harmony between the climate and the fauna and flora, the perfect adaptation of the one to the other.

That the atmosphere was highly charged with carbonic acid is rendered probable by the astonishing luxuriance of the vegetation of the period. Some experiments recently made by Mr. Gladstone seem to show that up to a certain limit the growth of ferns is rendered more rapid by the addition of carbonic acid to the atmosphere in which they grow. This probably becomes a certainty, when we reflect upon the enormous amount of carbon contained in the coal deposits, all of which must have been extracted from the atmosphere. It has been estimated that "all the forests of the United States gathered into one heap would fail to furnish materials of a single coal seam equal to that of Pittsburg." Again, that “that there is laid up in the earth, in the form of coal, six times as much carbon as now exists in the atmosphere. If it was all returned to the air, there would be seven times as much carbonic acid in the atmosphere as at present.

Cause of the climate of the coal.- Much speculative ingenuity has been exhausted to little effect in attempts to account for the remarkable climate of this period. We find here the same looseness of reasoning unfortunately so common among geologists when dealing with physical subjects. The subject of most of this speculation has been the cause of the supposed greater heat of the climate. There are two principal methods of accounting for it. The first and most obvious mode is by means of the commonly received hypothesis that the earth has cooled down to its present temperature from an original state of incandescence. But although there is much independent evidence of this original condition and we think it extremely probable, therefore, that the heat of the coal period was due, at least in part, to this causeyet, as Hopkins has shown, (Geol. Jour., 1853,) there are strong objections to this as the only cause. We have already said that the surface temperature of the earth is due partly to internal and partly to external causes. At present the surface temperature from internal causes has become almost nothing, i. e, only one-twentieth of a degree Fahrenheit. The increase of temperature below the surface is about 1° to sixty feet. Now, if we supposed the surface temperature from this cause to be increased even to 1°, the increase for every sixty feet, of depth would be 20o. An increase of 10° surface temperature would make 200° increase of temperature for every sixty feet. The springs, except the most superficial, would all be boiling. Now, it will be recollected that the winter temperature of Mellville, where coal is found abundantly, is —20° Fahrenheit. It would, therefore, take near

« AnteriorContinuar »