But these Ages are themselves susceptible of subdivisions, which should also be accurately defined. What is the nature of these subdivisions? They are all connected with sudden physical changes in the earth's surface, more or less limited in their action, these changes being themselves related to important alterations in the organic world. Although I have stated that one general character prevailed during each of the Ages, yet there was nevertheless a constant progressive action running through them all, and at various intervals both the organic and the physical world received a sudden impulse in consequence of marked and violent changes in the earth-crust, bringing up new elevations, while at the same time the existing animal creation was brought to a close, and a new set of beings was introduced. These changes are not yet accurately defined in America, because the age of her mountains is not known with sufficient accuracy; but their limits have been very extensively traced in Europe, and this coincidence of the various upheavals with the introduction of a new population differing entirely from, the preceding one has been demonstrated so clearly that it may be considered as an ascertained law. What name, then, is most appropriate for the divisions thus marked by sudden and violent changes? It seems to me, from their generally accepted meaning, that the word Epoch or Era, both of which have been widely, though indiscriminately, used in geology, is especially applicable here. In their common use, they imply a condition of things determined by some decisive event. In speaking of human affairs, we say, "It was an epoch or an era in history,"—or in a more limited sense, "It was an epoch in the life of such or such a man." It at once conveys the idea of an important change connected with or brought about by some striking occurrence. Such were those divisions in the history of the earth when a violent convulsion in the surface of the globe and a change in its inhabitants ushered in a new aspect of things.
I have said that we owe to Élie de Beaumont the discovery of this connection between the successive upheavals and the different sets of animals and plants which have followed each other on the globe. We have seen in the preceding article upon the formation of mountains, that the dislocations thus produced show the interruptions between successive deposits: as, for instance, where certain strata are raised upon the sides of a mountain, while other strata rest unconformably, as it is called, above them at its base,—this term, unconformable, signifying merely that the two sets of strata are placed at an entirely different angle, and must therefore belong to two distinct sets of deposits. But there are two series of geological facts connected with this result which are often confounded, though they arise from very different causes. One is that described above, in which a certain series of beds having been raised out of their natural horizontal position, another series has been deposited upon them, thus resting unconformably above. The other is where, one set of beds having been deposited over any given region, at a later time, in consequence of a recession of the sea-shore, for instance, or of some other gradual disturbance of the surface, the next set of beds accumulated above them cover a somewhat different area, and are therefore not conformable with the first, though parallel with them. This difference, however slight, is sufficient to show that some shifting of the ground on which they were accumulated must have taken place between the two series of deposits.
This distinction must not be confounded with that made by Élie de Beaumont: we owe it to D'Orbigny, who first pointed out the importance of distinguishing the dislocations produced by gradual movements of the earth from those caused by mountain-upheavals. The former are much more numerous than the latter, and in every epoch geologists have distinguished a number of such changes in the surface of the earth, accompanied by the introduction of a new set of animals, though the changes in the organic world are not so striking as those which coincide with the mountain-upheavals. Still, to the eye of the geologist they are quite as distinct, though less evident to the ordinary observer. To these divisions it seems to me that the name of Period is rightly applied, because they seem to have been brought about by the steady action of time, and by gradual changes, rather than by any sudden or violent convulsion.
It was my good fortune to be in some degree connected with the investigations respecting the limitation of Periods, for which the geology of Switzerland afforded peculiar facilities. My early home was near the foot of the Jura, where I constantly faced its rounded domes, and the slope by which they gently descend to the plain of Switzerland. I have heard it said that there is something monotonous in the continuous undulations of this range, so different from the opposite one of the Alps. But I think it is only by contrast that it seems wanting in vigor and picturesqueness; and those who live in its neighborhood become very much attached to the more peaceful character of its scenery. Perhaps my readers will pardon the digression, if I interrupt our geological discussion for a moment, to offer them a word of advice, though it be uncalled for. I have often been asked by friends who were intending to go to Europe what is the most favorable time in the day and the best road to enter Switzerland in order to have at once the finest impression of the mountains. My answer is always,—Enter it in the afternoon over the Jura. If you are fortunate, and have one of the bright, soft afternoons that sometimes show the Alps in their full beauty, as you descend the slope of the Jura, from which you command the whole panorama of the opposite range, you may see, as the day dies, the last shadow pass with strange rapidity from peak to peak of the Alpine summits. The passage is so rapid, so sudden, as the shadow vanishes from one height and appears on the next, that it seems like the step of some living spirit of the mountains. Then, as the sun sinks, it sheds a brilliant glow across them, and upon that follows—strangest effect of all—a sudden pallor, an ashy paleness on the mountains, that has a ghastly, chilly look. But this is not their last aspect: after the sun has vanished out of sight, in place of the glory of his departure, and of the corpse-like pallor which succeeded it, there spreads over the mountains a faint blush that dies gradually into the night. These changes—the glory, the death, the soft succeeding life—really seem like something that has a spiritual existence. While, however, I counsel my friends to see the Alps for the first time in the afternoon, if possible, I do not promise them that the hour will bring with it such a scene as I have tried to describe. Perfect sunsets are rare in any land; but, nevertheless, I would advise travellers to choose the latter half of the day and a road over the Jura for their entrance into Switzerland.[3]
It was from the Jura itself that one of the great epochs in the history of the globe received its name. It was in a deep gorge of the Jura, that, more than half a century ago, Leopold von Buch first perceived the mode of formation of mountains; and it was at the foot of the Jura, in the neighborhood of Neufchatel, that the investigations were made which first led to the recognition of the changes connected with the Periods. As I shall have occasion hereafter to enter into this subject more at length, I will only allude briefly here to the circumstances. In so doing I am anticipating the true geological order, because I must treat of the Jurassic and Cretaceous deposits, which are still far in advance of us; but as it was by the study of these deposits that the circumscription of the Periods, as I have defined them above, was first ascertained, I must allude to them in this connection.
Facing the range of the Jura from the Lake of Neufchatel, there seems to be but one uninterrupted slope by which it descends to the shore of the lake. It will, however, be noticed by the most careless observer that this slope is divided by the difference in vegetation into two strongly marked bands of color: the lower and more gradual descent being of a lighter green, while the upper portion is covered by the deeper hue of the forest-trees, the Beeches, Birches, Maples, etc., above which come the Pines. When the vegetation is fully expanded, this marked division along the whole side of the range into two broad bands of green, the lighter below and the darker above, becomes very striking. The lighter band represents the cultivated portion of the slope, the vineyards, the farms, the orchards, covering the gentler, more gradual part of the descent; and the whole of this cultivated tract, stretching a hundred miles east and west, belongs to the Cretaceous epoch. The upper slope of the range, where the forest-growth comes in, is Jurassic. Facing the range, you do not, as I have said, perceive any difference in the angle of inclination; but the border-line between the two bands of green does in fact mark the point at which the Cretaceous beds abut with a gentler slope against the Jurassic strata, which continue their sharper descent, and are lost to view beneath them.
This is one of the instances in which the contact of two epochs is most directly traced. There is no question, from the relation of the deposits, that the Jura in its upheaval carried with it the strata previously accumulated. At its base there was then no lake, but an extensive stretch of ocean; for the whole plain of Switzerland was under water, and many thousand years elapsed before the Alps arose to set a new boundary to the sea and inclose that inland sheet of water, gradually to be filled up by more modern accumulations, and transformed into the fertile plain which now lies between the Jura and the Alps. If the reader will for a moment transport himself in imagination to the time when the southern side of the Jurassic range sloped directly down to the ocean, he will easily understand how this second series of deposits was collected at its base, as materials are collected now along any sea-shore. They must, of course, have been accumulated horizontally, since no loose materials could keep their place even at so moderate an angle as that of the present lower slope of the range; but we shall see hereafter that there were many subsequent perturbations of this region, and that these Cretaceous deposits, after they had become consolidated, were raised by later upheavals from their original position to that which they now occupy on the lower slope of the Jura, resting immediately, but in geological language unconformably, against it. The two adjoining wood-cuts are merely theoretical, showing by lines the past and the present relation of these deposits; but they may assist the reader to understand my meaning.
Figure 1 represents the Jura before the Alps were raised, with the Cretaceous deposits accumulating beneath the sea at its base. The line marked S indicates the ocean-level; the letter c, the Cretaceous deposits; the letter j, the Jurassic strata, lifted on the side of the mountain.
