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

we were to calculate the quantity of carbon in all the carboniferous deposits at two thousand billion pounds (a quantity which must be much under the truth); then during its formation no less than 64,000,000,000,000,000 cubic feet of carbonic acid must have been extracted from the atmosphere, and a like quantity of oxygen gas returned to it.* This is equal to of the quantity of carbonic acid present in the whole extent of the atmosphere. And when we consider that this is but a portion of the carbonic acid removed, we may reasonably conclude that the atmosphere contained, at the commencement of the great carboniferous epoch, more than double the quantity of carbonic acid which it does now.

We have no grounds for affirming that there is a less vegetation now than in early times. On the contrary, it is highly probable that the vegetation now is much greater than that of former periods; but it is no less certain that the vegetation of former times was vastly more luxuriant at given places. For the continents being of smaller dimensions, more carbonic acid could be spared to support a luxuriant vegetation in a confined area. It is owing to this great luxuriance of vegetation, within a limited district, that vegetable remains were accumulated in such quantity as to defy even a remote analogy at the present day.

Without reference to geological epochs, I may here state in what manner coal and lignites are produced. The two principal kinds of coal are, the wood, or brown coal of Germany, and the stone, or mineral coal so abundantly found in our own country.

The wood coal, from its composition, has evidently been formed by a regular decay of plants with limited access of air. Hence the hydrogen is still present, whilst the oxygen has disappeared along with carbonic acid. Mineral coal, on the other hand, is distinguished from wood coal by containing a very small portion of hydrogen. Wood coal has been formed by the evolution of carbonic acid from the sub

1000 lbs of charcoal in burning produce above 32,000 cubic feet of carbonic acid. 1000=32,000, or 1=32: 2,000,000,000,000,000 lbs. will produce 64,000,000,000,000,000 cubic feet.

2,000,000,000,000,000 lbs.

2240 lbs.

892,857,142,857 tons.

But the assumed number, 2,000,000,000,000,000, is empirical, and we have, therefore, to shew it is not above the truth, however far it may be below it. Now we have already seen that Manchester by fuel, for domestic purposes alone, sends into the air, every year, 23,614,285,714 cubic feet of carbonic acid. And, taking its manufactories into the calculation, we may safely suppose, that the total amount will not be less than 46,000,000,000. Now1,391,304

64,000,000,000,000,000

46,000,000,000

That is, Manchester would consume the total amount we have supposed to exist in 1,391,304 years. Or supposing that there were in the world about 65,000 places consuming the same amount of fuel as Manchester, the total amount of coal in the great carboniferous deposits would be consumed in about twenty-one years. But this is obviously absurd, for we know that there is a much greater supply than this. Hence our original empirical number, instead of being above, must be much under the truth.

stance of the plants composing it; whilst mineral coal has been formed from the expulsion of part of its elements in the form of combustible oils. These oils may often be procured from the coal by distillation. Heat appears to have been the cause of the expulsion of these oils. A remarkable example occurs in a quarry within a few miles of St. Andrews, between that town and Cupar. A basaltic rock which has penetrated through the carboniferous strata, forms a hill in the locality alluded to: this rock is thoroughly impregnated with coal naptha. At whatever part a fragment may be broken off, the fresh surfaces are quite humid with an imprisoned fluid, which almost instantaneously evaporates; this fluid has the smell and all the properties of coal naptha. The great difference, therefore, in the formation of wood and mineral coal is, that in the production of the former, carbonic acid is evolved; in that of the latter a hydrocarbon. Hence it is that no combustible gases exist in the mines of wood coals, whilst they abound in those of mineral.

But be this as it may, our conclusion is still the same: that during the formation of the carboniferous deposits, much carbonic acid was abstracted from, and much oxygen furnished to the atmosphere. From the very low numbers which we assumed as indicating the weight of coal in the carboniferous strata, we have seen that its conversion into carbonic acid would nearly double the quantity of that gas now in the atmosphere. But the marine plants, which probably abounded in the same proportion as the terrestrial, have left few evidences of their former existence. They are so perishable in their nature, and surrounded by an element which aids their decay, that their preservation was highly improbable. But during their decay,

the carbonic acid from which they were formed, must have been given to the surrounding water; and probably entering into chemical combination with some of its materials, was not again restored to the atmosphere. From whence came all the carbonic acid in the limestones not formed by the accumulation of shells? Some of it, certainly, may have been derived from the source just mentioned. Nor are we to allow ourselves to be misled by the belief that the quantity of carbonic acid evolved from such a source, would be too small to exercise an appreciable effect. The decomposing organic matter has perceptibly affected the whole mass of the ocean in its vast extent; for all the recent analyses of sea water prove the presence of sulphuretted hydrogen-a gas only generated by the action of decomposing organic matter on salts of sulphuric acid. There are salts of lime in sea water, particularly the sulphate of lime, now this salt is very easily decomposed by a carbonate. Supposing that during the decay of the marine plants, which every analogy leads us to suppose must have existed in quantity proportional to the terrestrial, an alkaline carbonate was produced: this acting upon the sulphate of lime would occasion a precipitation of carbonate of lime, and give rise to those soluble alkaline sulphates, which we find in such quantity in sea water. By this suggested explanation, I by no

means infer that this was a general mode by which the stratified limestone was produced: the undivided limestone could not possibly have been produced in this way. But it is possible the thin layers of limestone which occasionally alternate with the shale, sandstone or coal, in the coal formation, may be due to such a cause; or might we not conceive that the bituminous limestone shale might also owe its production to this? The immense mass of undivided mountain limestone could by no possibility have been thus produced, but may have well been by the deposition from solution through the instrumentality of the causes I have formerly described. Once allow, with many geologists, that the ocean was in a heated state during a great part of the primary period, and we are furnished with another mighty means of abstracting the carbonic acid from the atmosphere. The heated waters of the ocean could not dissolve carbonic acid, but as they cooled, this gas would be absorbed from the superincumbent air; and the water which evaporated and again descended as rain, would bring down in solution large quantities both of carbonic acid and ammonia.

Taking such things as these into consideration, together with their possibility or probability, it is obvious that we have lost all data for calculating the amount of carbonic acid in the air, at the commencement of the secondary period. Allowing them even a shade of probability, we could not deny that the former atmosphere may have contained more than twenty times the amount of carbonic acid that it does at present; and admitting that it did so, we can account for the extraordinary luxuriance of the primeval vegetation and for the absence of land animals whilst that vegetation lasted.

During the period at which the carboniferous strata were deposited, neither reptiles, birds, nor mammalia appear to have existed: nor was it possible that they could have existed, were these views of the state of the atmosphere correct.

It is not my intention to detain you, nor is it my province to wander with you step by step over the various geological epochs. In our brief sojourn in the carboniferous strata, we have seen that several, possibly many causes were in operation to remove carbonic acid from the air, and consequently to fit it for the support of animal life. But let us not suppose that these causes ceased with the termination of the carboniferous era. They still operated, though in a less striking degree during all the divisions of the secondary period; but, during the deposition of the new red sandstone, they appear to have been in a great measure dormant. It is the duty of the geologist to explain what physical causes then existed, which were so unfavourable to animal and vegetable life. But the causes which acted during the carboniferous period were again revived with the oolitic system; and, accordingly, from the coal occurring in it, we draw evidences of a removal of carbonic acid from the atmosphere, and a supply of oxygen to it. But here also we are struck with the new forms of animal life which have now sprung into existence: the