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-land inclosed between the Rocky Mountains and the coast-range on 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 levelled 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 rivalled 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 local action of fresh water in depositing materials, afterwards traced by Cuvier and Brogniart 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 basalts to have been deposited in the ocean and crystallized from the substances it contained in solution.
But, in the mean time, 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 tilling 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 views, which were nevertheless the result of the stimulus he himself had given to geological investigations.