There can be no doubt that, on any supposable case, these lines are of very high antiquity. Now, wherever they are found on similar slopes, on similar ground, or generally in the same circumstances, they present such resemblances as to entitle us to conclude, that had the ground been uniform everywhere, they would have been everywhere equal and similar. Yet, as they lie at differ ent elevations, they are unequally subject to the action of the chief causes of waste, namely, the descent of water along the slopes of the hills, and they should, therefore, have shown distinct and different marks of waste, had these causes been of an active nature. Had such causes, indeed, been very active, they must have been obliterated, instead of having suffered, as they have done, so little in jury ; for, if they had suffered much, that waste, in the doctrine of chances, must have been unequal. We may, therefore, consider them as differing but little, even at this distant period, from the condition in which they were left by the subsidence•of the water, or by the cessation of the generating causes. The general conclusion from this is, that the waste and descent of hills, howeve certain it may be, is a very tedious operation. We are here furnished with certain unknown limits, but very wide ones we are sure, within which we can estimate that little waste has been suffered by the sides or summits of the hills of Glen Roy.
Some other important facts respecting the alluvia arc also deducible from these appearances. It has been re marked, that the lines are formed in two distinct sets of alluvia. The one of these occurs at the upper part of the glen, and it consists of sharp fragments that have been subjected to no distant transportation. These arc the re sult of the tedious process of waste acting on the sum mits of the hills, as is proved by their identity with the natural rock, by their irregular forms, and by other cir cumstances. The chief of these last is the intermixture of clay with the fragments; an important character be longing peculiarly to these alluvia of descent, as they may be called. Transported alluvia, besides that their frag ments are rounded and heterogeneous, alternate; the larger pieces, the gravel and the sand, forming a kind of distinct strata. If clay should happen to be present, it is disposed in a similarly separate manner, and not inter mingled with the coarser materials. Now, if we consider the case of the untransported alluvia, or those which co ver the faces of the hills, it is evident that a great length of time must have been required for their accumulation before the lines were formed in them. We have also seen, that, from the very little change these lines have under gone since their formation, scarcely any wearing of the hills has occurred, or any deposition of fresh alluvia of descent formed since the period at which they were traced, distant as that may be. Thus, we are necessarily carried back to a remote antiquity indeed, previous to the forma tion of these lines, for the deposition of the great quantity of alluvia which form such a thick covering on the hills, and which must have been produced during a very long period prior to the drainage of the lakes.
The following reasoning is no Idss intricate, and the consequences no less remarkable : As the alluvia of de scent which form the lines in the upper part of the valley are not much rounded, it is plain that no more violent mo tion than that which usually attends the margin of lakes had there existed in these waters. Neither, of course,
could there be any such motion in the lower parts of the valley, since the circumstances must have been the same throughout. Yet the lines in these places are formed in a rounded and transported alluvium of pebbles, sand, and gravel. Now, we know not how such an alluvium could have been thus accumulated, except from the action of waters flowing through this valley, and this accumulation must have been formed before the presence of the lake. Thus we are carried back to a far distant time, and to a state of the valley when it contained no water, and prior even, perhaps, to the very remote time in which the al luvia of descent were deposited on the hill faces. This reasoning, it will be perceived, confirms that formerly de duced from the appearances of the furrows, however un willing we may be to contemplate results so extraordinary and intricate.
Such are a few of the complicated conclusions which may be drawn from a proper contemplation of phenomena that must not be looked at by a careless eye, and that can not be advantageously stated except by geologists well versed in all the complicated appearances and niceties which belong to the past and present state of the surface, and quick in seizing on the very delicate relations which belong to them. We know of no spot which requires such steadiness and minuteness of attention; and which demands so many processes of reasoning before we can derive from it all the advantages which its study is calcu lated to yield. Wc are not aware that any of the conclu sions which we have here attempted to draw from them are overstrained, far less groundless. Yet, though some of them should be founded in error, there is still enough to excite our industry in the observation of similar pheno mena, and in seeking for farther analogies, where to dis cover appearances exactly similar seems hopeless.
Of the probability of the theory here offered we need say no more ; that it is simple and explanatory is certain, but that it implies some circumstances difficult to explain we have already shown. These, however, arc explained by no other hypothesis ; while each of those that has been proposed, and, indeed, all others of this nature that can possibly be conceived, are attended with difficulties far more complicated, and even imply impossibilities, a fault from which the theory that we have adopted is assuredly free. We have only yet to hope, that a farther acquaint ance with the general changes which the surface of the earth has undergone, with the causes of them, and, per haps, the future discovery of some analogies or resem blances to these phenomena, will ultimately remove that which yet remains difficult of explanation in the physical history of Glen Roy, and of its Parallel Roads.
The study to which we have alluded is difficult ; it re quires an eye, and a habit of reasoning, far different from those which are engaged in the comparatively trifling business of ascertaining the names and positions of rocks, dnd the nature of minerals. Let us hope that those who really have the requisite faculties will bestow more at tention on this important and difficult branch of Geology ; and that the history of the surface of the earth, and its changes, may one day be rescued from the state in which it is at present.