OCR Text |
Show (4. ERA OF THE CARBONIFEROUS FORMATION. have a comparati \rely brief period of volc.anic disturba:.1ce (when the conglomerate was formed.) Then the causes favorable tQ the so abundant production of limestone, and the large population of marine acrita, decline, and we finrl the rnaRses of dry land increase in number and extent, and. begin to bear an amount of forest vegetation, far exceed· i.ng that of the most sheHered tropical spots of the present surface. The climate, even in the latitude of Baffin's Bay, was torrid, and perhaps the atmosphere contained alarger charge of cat·bonic acid gas (the material of vegetation) than it no\v does. The forests or thickets of the period, included no species of plants now known upon earth. They mainly consisted of gigantic shrubs, which are either not represented by any existing types, or are akin to kinds \vhich are now only found in small and lowly forms. That these forests grew upon a Polynesia, or multitude of small islands, is considered probable, from similar vegetation being now found in such situations within the tropics. With regard to the circumstances under which the masses of vegetable n1atter were transformed into successive coal strata, geologists are dividetl. From exam .. ples seen at the present day, at the mouths of such rivers as the Mississippi, which traverse extensive sylvan regions, and from othet· circumstances to be adverted to, it is held likely by some that the vegetable matter, the rubbish of decayed forests, was carried by rivers into estuaries, and there accumulated in vast natural rafts, until it sunk to the bottom where an overlayer of sand or mud would prepare it for becoming a stratum of coal. Others conceive that the vegetation first went into the condition of a peat-moss, that a sink in the level then exposed it to be overrun by the sea, and covered with a layer of sand or mud; that a subseq~ent uprise made the mud dry land, ilnd fitted it to bear a new forest, which afterwards, like its predecessor, became a bed of peat; that, in short, by repe· titions of thi~ process, the alternate layers of coal, sandstone: and shale, constituting the carboniferous group, were formed. It is favorable to this last view that marine fossils are scarcely found in the body of the coal itself, though abundant in the shale layers above and below it; also that in several places erect stems of trees are found with their root~ still fixed in the shale beds, and crossing the sandttone beds at almost right angles, showing that these, at least, had not been drifted from their uriginal situationi COMMENCEMENT OF LAND 1-LA.NTS. ~n. the other. ha.nd, it is not easy to admit such repeated r~tngs and Sinkings of surface as would be required on this hypothe~Is, to form a series of.coal strata. Per hap; we may most saiely rest at present with the supposition that coal has. been formed under both classes of circumstances, though In the latter only as an exception to the former U pw.ards of thi~ee hundred species of plants have bee~ ascertained to exist In the coal formation; but it is not necessary to suppose that the whole contained in that systern are now, or will be, distinguished. Experiments sho.w that so.me great classes of plants beoome decompos~ d In water In a much less space of time than others and It 1s remarkable that those which decompose soonest' are of the class~s found most rare, or not at all, in the 'coal strata. It IS consequently to be inferred that there may h!lve been grasses and mosses at this era and many speCies of trees, the remains of which had iost all trace ot org~ntc form before their substance sunk into the mass ot W~1Ch coal _was f~und. In speaking, therefore of the vegetatiOn of this penod, we must bear in mind that itmay have comprehe_nded forms of which we have no memorial .Supposing, ~everthcless, that, in the main, the ascertained ':'egetab.on of the .coal. s~ste.m is that which grew at the tune .of Its formation, It Is Interesting to find that the terres.tnal b?tany of our globe begins with classes of comparatively simp~e forms and structure. In the ranks of the vegetable kt.ngdom, the lowest place is taken by plants of cellular tissue, and which have no flowers (cbyptoga1nia,) as lichens, mosses, fungi, ferns, sea-weeds~ A ove t~ese st~nd pl~nts of vascular tissue, and beariug ~owers, In which again there are two great subdivisions· .H·st, ~!ants having one s~ed-lobe (monocotyledons,) and In wh.ICh the new matter IS added within (endogenous,) of "~hich the cane and palm are examples; second plants having two s~ed-lobes (dycotyledons,) and in which the new n1atter 1s added on the outside under the bark he~ogenous,) of which the pine, elm, oak, and most of th~ nh~h ~orest-trees are examples; these subdiYisions also ~a~long In the order in which they are here stated Now I~ Is clear that a predominance of these forms in ·succesSIOn ~a~ked tl~e successive . epochs developed by fossil gatieolo~y, the simple abounding first, and the complex terwards. Two-thirds of the plants of the carboniferous era are |