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cess carbonate of lime will not dissolve. Now, without entering into geological speculations, it seems to be pretty conclusively established, that the heat of the ocean was considerable during the deposition of the earlier of the primary strata.
From the contortions in the laminæ of greiss, mica schist, and chlorite schist, the water from whence they were deposited appears to have been in a very troubled state, just as it is in the present day when in a hot vessel. Now, when in this hot state it could not absorb or retain in solution carbonic acid from the air. Only in those spots where local causes had reduced the heat, carbonic acid might be dissolved, and carbonate of lime formed; or, if pre-existing, brought into solution, and afterwards deposited. This heated state of the ocean might, in some degree, account for its saline impregnation. There is no evidence of land having existed during the primary period, and during the ages which constituted it, this heated water acting upon the rocks which formed the crust of the globe, would have a wonderful effect in promoting chemical combinations, and dissolving the various soluble materials contained in these rocks. In all strata of later origin than the primary, we find rock salt. Salt springs arise from the coal system-rock salt abounds in the new red sandstone. The Alpine salt works are in the colitic system; those of Cardona in the greensand; and of Wieliczka in the tertiary rocks. (Phillips). But, as far as I am aware, rock salt is absent from the primary strata. This seems to indicate that the water had not obtained its full saline impregnation when those strata were deposited. And indeed, it seems not improbable, that during this era, the water obtained many of its soluble salts from the disintegration of the primary rocks.
But to return from our digression. The existence of the limestones proves that even in the primary period, carbonic acid existed. The peculiar smell which distinguishes hornblende, and various aluminous minerals, when they are moistened, is due to minute traces of ammonia contained in them. Now this smell is very perceptibly possessed by the hornblende schists, which accompany gneiss; and hence we have a right to conclude, that they absorbed this compound from the air, or from the water, during their deposition. There are many facts to prove that ammonia was formerly of inorganic origin. Not only is it a constituent of all aluminous and ferruginous minerals, but it exists in many natural products found in volcanic regions. Dr. Daubeny has supposed that all the carbonic acid and ammonia which now exists, or has existed in the atmosphere, may have been derived from the interior of the earth. He finds a difficulty in conceiving how the hydrogen and nitrogen could have been made to unite on the surface of the globe, and hence, draws them from its interior. But although we do not agree as to the source from whence the food of plants is derived, we both equally admit that the primary food was of inorganic origin.
But I must assume, for the present, on the supposition that these
bodies were original constituents of the atmosphere, that they both existed in the air, during the primary periods, or, at all events, antecedent to the secondary, in much larger quantity than they do now : and the proofs are obvious. Consider the immense deposits of vegetable matter in the carboniferous strata. All the carbon and nitrogen of which this is composed must have been originally present in the air as carbonic acid and ammonia. The greatest part of this vegetable matter is carbon ; and hence it follows, that, as the carbon of the carbonic acid was retained, and its oxygen liberated, the air of the present day must be much poorer in carbonic acid, but much richer in oxygen, than that of former ages. Dr. Daubeny denies the possibility of ammonia ever having been present in the air in much larger quantity than at present; and, in proof of his view, he cites the experiments of Christison and Turner, that lộ of ammoniacal gas in air acts as a poison to the vegetable kingdom. To this it may be replied, that ammoniacal gas is never present in the atmosphere. It is always in union with carbonic acid as carbonate of ammonia ; and it is well known that water impregnated with this salt in the proportion of 30 : 1, is very beneficial to vegetation. One pound of rain water contains rather less than 1 grain of carbonate of ammonia. Now, even supposing that the rain of former ages contained 800 times this quantity, it would not be prejudicial to plants, but administer to the luxuriance of vegetation. Again, it has been found that plants flourish with great luxuriance in an atmosphere containing as much as its bulk of carbonic acid. But the air of the present day contains only one volume of this gas in 2,000 volumes of air. Hence it follows, that the air of former ages may have contained more than 150 times this amount, without injury to vegetation. These facts are sufficient to prove that the air formerly may have been much richer in the food of plants. Brogniart believed this to be the case many years since, and founded upon this view some ingenious speculations. Doubtless an atmosphere with quantities such as we have mentioned, would prove fatal to animal life, but we will shortly show that the express duty of former plants was to prepare the world for the reception of animals, and finally, for that of an intellectual being. One question, then, only remains : from whence did the atmosphere receive this carbonic acid ? Dr. Daubeny conceives by a gradual evolution from the interior of the earth. He supposes that this carbonic acid was furnished to plants during their life-not that a certain amount was originally emitted into the great magazine of food—the atmosphere. But it is difficult to conceive that an All-wise Creator would have made the life of plants and animals dependant upon adventitious circumstances. And surely it cannot be averrred that the emission of carbonic acid from volcanic sources, would be regulated by a direct interposition of a Divine Providence. The vegetation of the globe did not at once spring into existence, but was as slow in its progression, as that of animals ; so that during the ages, when a scanty vegetation
covered the earth, the carbonic acid must have been constantly accumulating. And if, at its commencement, there was sufficient carbonic acid for the wants of plants, in the course of ages it must have been accumulated in much larger quantities. So that even on this view we must admit, that the proportion in the air could not be kept in any constant quantity. But we do not see any strong reason to suppose that the original food of plants was evolved from the bowels of the earth ; nor can we discover any strong objection to the hypothesis, that all the carbonic acid and ammonia from the beginning of time, were original constituents of the atmosphere. The nitrogen and oxygen of which the atmosphere consists were certainly not evolved from the interior of the earth; and yet upon the same grounds could this be affirmed, for nitrogen, like carbonic acid, is evolved from the volcanoes of the present day. It is indeed difficult to conceive from what stupendous magazines of carbon the carbonic acid of the air was formed, or how hydrogen became united with oxygen to form all the water which covers the earth, without the occurrence of such dreadful explosions as would dash the the present system of the world to fragments. Such stupendous operations of nature are as yet beyond the capacity of the human mind to fathom : just so with the ammonia. If we cannot comprehend the mighty power which deprived its elements of their elastic condition, we can only believe that that power is as yet undiscovered.
From all that has preceded we believe we are warranted in considering that the carbonic acid and ammonia in the air were original constituents of the atmosphere; and further, that in former ages, they were present in much larger proportion than at the present time. We have shewn that such an atmosphere would not be unsuitable for vegetable life, although it would, certainly, prove destructive to terrestrial animals. We have now to examine the economy of vegetable and animal life of former times.
The first certain proofs of organic life occur in the grauwacke series. It is certainly singular that animals occur in this series, where plants are not found. These animals are not merely zoophyta, but conchifera. Still we cannot suppose that sea weed did not exist to supply them with food; for the spongy texture of such plants make them prone to decomposition. The paucity of animal remains at this period, and their very unequal distribution, indicate that the conditions necessary for the support of animal life were not yet generally established. As we reach the silurian rocks we discover that the conditions for organic life have become more favourable, and consequently we find a much greater variety of forms; but we are still struck by the paucity of vegetable remains. Doubtless these were present in quantity sufficient to supply food to the marine animals which existed, but their perishable tissues have yielded to the elements of destruction and disappeared. The upheaved land during the grauwacke and silurian systems seems to
have been too limited to favour the production of terrestrial plants.
The disturbances which ensued after the close of the primary period, rendered the earth more adapted for vegetation. Land was upheaved, and the energetic causes then in operation must have materially assisted in effecting its disintegration. The time which elapsed between the close of the primary, and commencement of the secondary periods, would be employed in the formation of soils on this upheaved land. Soils formed from the detritus of the primary rocks would be eminently adapted for a luxuriant vegetation, such as existed during the deposition of the carboniferous strata.
But no means of removing the excess of carbonic acid of the air having been yet in operation, terrestrial plants could not be accompanied by terrestrial animals. It is obvious that this excess of carbonic acid could not be very detrimental to the life of marine animals; because the sea, saturated as it is with salts, can only hold a certain quantity in solution. But still the sea also must have contained considerably more of this gas then, than at the present day. The scantiness of vegetation was the great characteristic of the primary strata; but in the coal systems this vegetation is marvellous in its extent. The principal deposits in this series are arenaceous, argillaceous, and calcareous. The calcareous deposits of the primary period, even as high as the upper silurians, occur in detached masses, forming no continuous beds, like the carboniferous, or mountain limestones. We find a total absence of land reliquiæ in these calcareous beds. Add to this, that, besides the nature of their fossil remains, they afford evidences of a very tranquil and gradual deposition; and it is apparent they must be of marine origin. The excess of carbonic acid dissolved by the water from the atmosphere, would render the sea capable of retaining a large quantity of carbonate of lime in solution, and the luxuriant vegetation which covered the sea as well as the land, would constantly be abstracting this carbonic acid for the purposes of food, from the surrounding water. The carbonate of lime being thus rendered insoluble, would, be tranquilly deposited, and thus two substances being removed, which in excess are fatal to certain animals, other forms of organic life would spring into being. Here, then, we are furnished with an explanation why the limestone does not occur in continuous beds in former strata ; for however great the quantity of limestone in solution might have been, it could not have been deposited, were there no means of removing the carbonic acid which retained it in solution; only in particular localities, where adventitious circumstances occasioned the expulsion of carbonic acid, the limestone would be deposited.
And now we come to the consideration of those vast deposits of coal which form such striking monuments of a primeval vegetation. The entire coal series often attains a thickness of 1,000 yards; the beds of coal occurring in it are occasionally three or four feet thick, and not unfrequently several yards. The thickness of all the coal
beds taken together may average forty or fifty feet in the English and Scotch coal fields. Now when we consider the vast area covered by the coal series, we must feel convinced that during its formation, peculiar causes were in operation which occasioned a great luxuriance in vegetation. It is true that such rivers as the Oronoko and Missisippi roll down to the ocean vast quantities of vegetable matter ; but great as these are, they do not even furnish us with a faint conception of the manner in which the great carboniferous deposits have been formed. The wonderful luxuriance of vegetation during the carboniferous era is doubtless attributable to the amount of carbonic acid in the air. The remains of plants which constitute the various seams of coal, shew that they were principally terrestrial. Many of these beds of coal appear to have been formed of drift vegetation, but others shew every evidence of the plants having lived and died on the spot : this is the case with the North of England coal field, and most of the North American coal fields shew similar evidences ; the Devonshire coal fields, or culm measures are, on the other hand, I believe, frequently composed of drift matter.
Now in these coal fields which show evidences of having been formed in situ, a bed of fire clay is almost invariably found immediately below the coal. In this are present large quantities of stems and leaves of stigmaria, ficoides, &c. The constant occurrence of this underclay, evidently indicates some general cause. Now, when we examine the composition of the ashes of coal, and that of the fire clay, we discover the same ingredients in both. Potash, and magnesia are contained in the fire clay, and from their constant presence in coal appear to have been indispensable to the developement of the plants constituting it.
This underclay may then be viewed, with every probability, as the soil in which the plants grew; and the adaptation of such a soil to the plants was obviously due to its alkaline constituents. These primeval plants would not exhaust a soil so rapidly as those of the present day, for they invariably contain a much smaller quantity of inorganic ingredients; and their roots being but imperfectly developed would not furnish much excrementitious matter to the soil. Once admit that an excess of carbonic acid was in the air during this period, and an immense vegetation would be the result. The carbonic acid being once extracted could only be returned to the atmosphere by a complete decay of the plants which had used it as food. But we find that the plants constituting coal have been subjected only to partial decay. They have yielded up most of their oxygen, but their carbon has been for the most part retained : their hydrogen has also in a great measure disappeared. From the composition of coal compared with that of woody fibre, it is obvious that during the formation of 353 cubic feet of Newcastle splint coal, the atmosphere must have received 800 cubic feet of oxygen gas, and lost a corresponding quantity of carbonic acid. Now, suppose