THE FERN FORESTS OF THE CARBONIFEROUS PERIOD.
Draw two lines on your map, the upper one running from the mouth of the St. Lawrence westward nearly to St. Paul on the Mississippi, and the lower one from the neighborhood of St. John's in Newfoundland running southwesterly about to the point where the Wisconsin joins the Mississippi, but jutting down to form an extensive peninsula comprising part of the States of Indiana and Illinois, and you include between them all of the United States which existed at the close of the Devonian period. The upper line rests against the granite hills dividing the Silurian and Devonian deposits of the British Possessions to the north from those of the United States to the south, Canada itself consisting, in great part, of the granite ridge.
How far the early deposits extended to the north of the Laurentian Hills, as well as the outline of that portion of the continent in those times, remains still very problematical; but the investigations thus far undertaken in those regions would lead to the supposition that the same granite upheaval which raised Canada stretched northward in a broad, low ridge of land, widening in its upper part and extending to the neighborhood of Bathurst Inlet and King William's Island, while on either side of it to the east and west the Silurian and Devonian deposits extended far toward the present outlines of the continent.
Indeed, our geological surveys, as well as the information otherwise obtained concerning the primitive condition of North America and the gradual accessions it has received in more recent periods, point to a very early circumscription of the area which, in the course of time, was to become the continent we now inhabit, with its modern features.[A]
[Footnote A: It would be impossible to encumber the pages of the Atlantic Monthly with references to all the authorities on which such geological results rest. They are drawn from the various State Surveys, including that of the mineral lands of Lake Superior, and other more general works on American geology.]
Not only from the geology of America, but from that of Europe also, it would seem that the position of the continents was sketched out very early in the progressive development of the physical constitution of our earth. It is true that in the present state of our knowledge such wide generalizations must be taken with caution, and held in abeyance to the additional facts which future investigations may develop. But thus far the results certainly do not sustain the theories which have lately found favor among geologists, of entire changes in the relative distribution of land and sea and in the connection of continents with one another; on the contrary, it would appear, that, in accordance with the laws of all organic progress, arising from a fixed starting-point and proceeding through regular changes toward a well-defined end, the continents have grown steadily and consistently from the beginning, through successive accessions in a definite direction, to their present form and Organic correlations. If, indeed, there is any meaning in the remarkably symmetrical combinations of the double twin continents in the Eastern Hemisphere, so closely soldered in their northern half, as contrasted with the single pair in the Western Hemisphere, isolated in their position, but so strikingly similar in their Outlines, they must be the result of a progressive and predetermined growth already hinted at in the relative position and gradual increase of the first lands raised above the level of the ocean.
However this may be, there can be no doubt that we now know with tolerable accuracy the limits of the land raised above the water at that period in the present United States. Let us see, then, what we inclose between oar two lines. We have Newfoundland and Nova Scotia, the greater part of New England, the whole of New York, a narrow strip along the north of Ohio, a great part of Indiana and Illinois, and nearly the whole of Michigan and Wisconsin.
Within this region lie all the Great Lakes. The origin of these large troughs, holding such immense sheets of fresh water, remains still the subject of discussion and investigation among geologists. It has been supposed that in the primitive configuration of the globe, when the formation of those depressions at the poles in which the Arctic seas are accumulated gave rise to a corresponding protrusion at the equator, the curve thus produced throughout the North Temperate Zone may have forced up the Canada granite, and have caused, at the same time, those rents in the earth's surface now filled by the Canada lakes; and this view is sustained by the fact that there is a belt of lakes, among which, however, the Canada lakes are far the largest, all around the world in that latitude. The geological phenomena connected with all these lakes have not, however, been investigated with sufficient accuracy and detail, nor has there been any comparison of them extensive and comprehensive enough to justify the adoption of any theory respecting their origin. In an excursion to Lake Superior, some years since, I satisfied myself that the position and outline of that particular lake had their immediate cause in several distinct systems of dikes which intersect its northern shore, and have probably cut up the whole tract of rock over the space now filled by that wonderful sheet of fresh water in such a way as to destroy its continuity, to produce depressions, and gradually create the excavation which now forms the basin of the lake. How far the same causes have been effectual in producing the other large lakes I am unable to say, never having had the opportunity of studying their formation with the same care.
The existence of the numerous smaller lakes running north and south in the State of New York, as the Canandaigua, Seneca, Cayuga, etc., is more easily accounted for. Slow and gradual as was the process by which all that region was lifted above the ocean, it was, nevertheless, accompanied by powerful dislocations of the stratified deposits, as we shall see when we examine them with reference to the local phenomena connected with them. To these dislocations of the strata we owe the transverse cracks across the central part of New York, which needed only the addition of the fresh water poured into them by the rains to transform them into lakes.
I shall not attempt any account of the differences between the animals of the Devonian period and those of the Silurian period, because they consist of structural details difficult to present in a popular form and uninteresting to all but the professional naturalist. Suffice it to say, that, though the organic world had the same general character in these two closely allied periods, yet its representatives in each were specifically distinct, and their differences, however slight, are as constant and as definitely marked as those between more widely separated creations.