Page images

Such a

mountain side, we may have a most fertile vegetation at a great height, and surrounded by the dark pine forests. Many of the pastures on the Alps, to which from height to height the shepherds ascend with their flocks in the summer, -seeking the higher ones as the lower become dry and exhausted, -are due to such alternations in the character of the rocks.

In consequence of the influence of time, weather, atmospheric action of all kinds, the apparent relation of beds has often become so completely reversed that it is exceedingly difficult to trace their original relation. Take, for instance, the following case. An eruption has upheaved the strata over a given surface in such a manner as to lift them into a mountain, cracking open the upper beds, but leaving the lower ones unbroken. We have then a valley on a mountain summit between two crests. narrow passage between two crests may be changed in the course of time to a wide expansive valley by the action of the rains, frosts, and other disintegrating agents, and the relative position of the strata forming its walls may seem to be entirely changed.

Suppose, for example, that the two upper layers of the strata rent apart by the upheaval of the mountain are limestone and sandstone, while the third is clay and the fourth again limestone. Clay is soft, and yields very readily to the action of rain. In such a valley the edges of the strata forming its walls are of course exposed, and the clay formation will be the first to give way under the action of external influences. Gradually the rains wear away its substance till it is completely hollowed out. By the disintegration of the bed beneath them, the lime and sandstone layers above lose their support and crumble down, and this process goes on, the clay constantly wearing away, and the lime and sand above consequently falling in, till the upper beds have receded to a great distance, the valley has opened to a wide expanse instead of being inclosed between two walls, and the lowest limestone bed now occupies the highest position on the mountain.

But the phenomena of eruptions in mountain-chains are far more difficult to trace than the effects thus gradually produced. Plutonic action has, indeed, played the most fantastic tricks with the crust of the earth, which seems as plastic in the grasp of the

[ocr errors]

fiery power hidden within it as does clay in the hands of the sculptor.

We have seen that an equal vertical pressure from below produces a regular dome, - or that, if the dome be broken through, a granite crest is formed, with stratified materials resting against its slopes. But the pressure has often been oblique instead of vertical, and then the slope of the mountain is uneven, with a gradual ascent on one side and an abrupt wall on the other; or in some instances the pressure has been so lateral that the mountain is overturned and lies upon its side, and there are still other cases where one mountain has been tilted over and has fallen upon an adjoining one.

Sometimes, when beds have been torn asunder, one side of them has been forced up above the other; and there are even instances where one side of a mountain has been forced under the surface of the earth, while the other has remained above. Stratified beds of rock are occasionally found which have been so completely capsized, that the layers, which were of course deposited horizontally, now stand on end, side by side, in vertical rows. I remember, after a lecture on some of these extravagances in mountain formations, a friend said to me, not inaptly, -"One can hardly help thinking of these extraordinary contortions as a succession of frantic frolics; the mountains seem like a troop of rollicking boys, hunting one another in and out and up and down in a gigantic game of hide-andseek.”

The width of the arch of a mountain depends in a great degree on the thickness and flexibility of the beds of which it is composed. There is not only a great difference in the consistency of stratified material, but every variety in the thickness of the layers, from an inch, and even less, to those measuring from ten or twenty to one hundred feet and more in depth, without marked separation of the successive beds. This is accounted for by the frequent alternations of subsidence and upheaval; the continents having tilted sometimes in one direction, sometimes in another, so that in certain localities there has been much water and large deposits, while elsewhere the water was shallow and the deposits consequently less. Thin and flexible strata have been readily lifted into a sharp, abrupt arch with narrow base, while the thick and rigid beds have been forced up more slowly in a wider arch with broader base.

Table-lands are only long unbroken folds of the earth's surface, raised uniformly and in one direction. It is the same pressure from below, which, when acting with more intense force in one direction, makes a narrow and more abrupt fold, forming a mountain ridge, but, when acting over a wider surface with equal force, produces an extensive uniform elevation. If the pressure be strong enough, it will cause cracks and dislocations at the edges of such a gigantic fold, and then we have table-lands between two mountain-chains, like the Gobi in Asia between the Altai Mountains and the Himalayas, or the table-lands inclosed between the Rocky Mountains and the coast range of the Pacific shore.

We do not think of table-lands as mountainous elevations, because their broad, flat surfaces remind us of the level tracts of the earth; but some of the table-lands are nevertheless higher than many mountain-chains, as, for instance, the Gobi, which is higher than the Alleghanies, or the Jura, or the Scandinavian Alps. One of Humboldt's masterly generalizations was his estimate of the average thickness of the different continents, supposing their heights to be leveled and their depressions filled up, and he found that upon such an estimate Asia would be much higher than America, notwithstanding the great mountain-chains of the latter. The extensive table-land of Asia, with the mountains adjoining it, outweighed the Alleghanies, the Rocky Mountains, the Coast Chain, and the Andes.

When we compare the present state of our knowledge of geological phenomena with that which prevailed fifty years ago, it seems difficult to believe that so great and important a change can have been brought about in so short a time. It was on German soil and by German students that the foundation was laid for the modern science of systematic geology.

In the latter part of the eighteenth century, extensive mining operations in Saxony gave rise to an elaborate investigation of the soil for practical purposes. It was found that the rocks consisted of a succession of materials following each other in regular sequence, some of which were utterly worthless for industrial purposes, while others were exceedingly valuable. The

Muschel-Kalk formation, so called from its innumerable remains of shells, and a number of strata underlying it, must be penetrated before the miners reached the rich veins of Kupferschiefer (copper slate), and below this came what was termed the Todtliegende (dead weight), so called because it contained no serviceable materials for the useful arts, and had to be removed before the valuable beds of coal lying beneath it, and making the base of the series, could be reached. But while the workmen wrought at these successive layers of rock to see what they would yield for practical purposes, a man was watching their operations who considered the crust of the earth from quite another point of view.

Abraham Gottlob Werner was born more than a century ago in Upper Lusatia. His very infancy seemed to shadow forth his future studies, for his playthings were the minerals he found in his father's forge. At a suitable age he was placed at the mining school of Freiberg in Saxony, and having, when only twenty-four years of age, attracted attention in the scientific world by the publication of an “Essay on the Characters of Minerals,” he was soon after appointed to the professorship of mineralogy in Freiberg. His lot in life could not have fallen in a spot more advantageous for his special studies, and the enthusiasm with which he taught communicated itself to his pupils, many of whom became his devoted disciples, disseminating his views in their turn with a zeal which rivaled the master's ardor.

Werner took advantage of the mining operations going on in his neighborhood, the blasting, sinking of shafts, etc., to examine critically the composition of the rocks thus laid open, and the result of his analysis was the establishment of the Neptunic school of geology alluded to in a previous article, and so influential in science at the close of the eighteenth and the opening of the nineteenth century. From the general character of these rocks, as well as the number of marine shells contained in them, he convinced himself that the whole series, including the Coal, the Todtliegende, the Kupferschiefer, the Zechstein, the Red Sandstone, and the Muschel-Kalk, had been deposited under the agency of water, and were the work of the ocean.

Thus far he was right, with the exception that he did not include the accumulation of materials by the local action of fresh water afterwards traced by Cuvier and Brongniart in the Tertiary deposits about Paris. But from these data he went a step too far, and assumed that all rocks, except the modern lavas, must have been accumulated by the sea, - believing even the granites, porphyries, and the basalts to have been deposited in the ocean and crystallized from the substances it contained in solution.

But, in the meantime, James Hutton, a Scotch geologist, was looking at phenomena of a like character from a very different point of view. In the neighborhood of Edinburgh, where he lived, was an extensive region of trap-rock, – that is, of igneous rock, which had forced itself through the stratified deposits, sometimes spreading in a continuous sheet over large tracts, or splitting them open and filling all the interstices and cracks so formed. Thus he saw igneous rocks not only covering or underlying stratified deposits, but penetrating deep into their structure, forming dikes at right angles with them, and presenting, in short, all the phenomena belonging to volcanic rocks in contact with stratified materials. He again pushed his theory too far, and, inferring from the phenomena immediately about him that heat had been the chief agent in the formation of the earth's crust, he was inclined to believe that the stratified materials also were in part at least due to this cause. I have alluded in a former number to the hot disputes and long-contested battles of geologists upon this point. It was a pupil of Werner's who at last set at rest this much vexed question.

At the age of sixteen, in the year 1790, Leopold von Buch was placed under Werner's care at the mining school of Freiberg. Werner found him a pupil after his own heart. Warmly adopting his teacher's theory, he pursued his geological studies with the greatest ardor, and continued for some time under the immediate influence and guidance of the Freiberg professor. His university studies over, however, he began to pursue his investigations independently, and his geological excursions led him into Italy, where his confidence in the truth of Werner's theory began to be shaken. A subsequent visit to the region of extinct volcanoes in Auvergne, in the south of France, convinced him that the aqueous theory was at least partially wrong, and that fire had been an active agent in the rock formations of past times. This result did not change the convictions of his master, Werner, who was too old or too prejudiced to accept the later

« PreviousContinue »