161. The deposits described in the preceding paragraphs are either of vegetable or of animal origin; but there is an intimate admixture of both in the soil or superficial covering of the earth. Strictly speaking, soil is an admixture of decomposed vegetable and animal matter-the decay of plants, and the droppings and exuviæ of animals. Though generally containing a large proportion of earthy ingredients, its dark loamy aspect renders it readily separable from the "subsoil of sand, clay, or gravel, that lies beneath. It is of universal occurrence, no portion of the earth's crust being uncovered with it, unless, perhaps, the newly-deposited debris on the sea-shore, the shifting sands of the desert, or the snow-clad mountain-top. In some places it barely covers the flinty rock, in others it is several feet in thickness, and everywhere it is annually on the increase. " Igneous or Volcanic Accumulations. 162. The effects of igneous action in modifying the crust of the globe have been already adverted to in pars. 24-27, it having been there shown that it acts either in gradual crust-motions-that is, as a gradually elevating or depressing force-as a displacing and deranging force, or as an accumulating agent by discharges of lava, scoriæ, dust, and ashes. Whether manifesting itself in quiet upheavals, in earthquakes, or in volcanoes, its geographical results are of prime importance; and though in certain areas, the present epoch, as compared with some of the past, be one of rest and tranquillity, yet wide regions of the globe bear witness to extensive modifications even within the history of man. Volumes might be filled with the records of such changes; our limits only permit a few recent examples :-Since the commencement of the present century, the shores of the Baltic have been gradually elevated from eighteen to twenty-two inches above their former level, and are still apparently on the uprise. A similar uprise is taking place all along the shores of Siberia, Spitzbergen, and the Arctic islands of North America-the whole being marked by terrace above terrace to the height of several hundred feet. By an earthquake in 1819, a tract in the delta of the Indus, extending to nearly fifty miles in length and sixteen in breadth, was upheaved ten feet, while adjoining districts were depressed, and the features of the delta completely altered. By the great Chili earthquake of 1822, a tract of not less than one hundred thousand square miles was permanently elevated about six feet above its former level; and part of the sea-bottom remained dry at highwater, with beds of shell-fish adhering to the rocks on which they grew. Within the last ten or twelve years the shores of North Island, New Zealand, have been variously affected, and a large portion in 1856 suddenly but permanently upraised to the height of six or eight feet above its previous level. As with these instances of upheaval so with others of depression, like the western shores of Greenland, the southern shores of Norway, and the seaboard of the Southern States of North America-all of which are said to be undergoing a slow but continued submergence. 163. The above are examples of upheaval and depression on a great scale, and attended with comparatively few convulsions or displacements. The following are of a different order :-In 1692, the town of Port Royal in Jamaica was visited by an earthquake, when the whole island was frightfully convulsed, and about a thousand acres in the vicinity of the town submerged to the depth of fifty feet, burying the inhabitants, their houses, and the shipping in the harbour. The disasters of the great Lisbon earthquake in 1755, when the greater part of that city was destroyed, and sixty thousand persons perished in the course of a few minutes, have been repeatedly recited; as have also those of Calabria, which lasted nearly four years-from 1783 to the end of 1786-producing fissures, ravines, landslips, falls of the sea-cliff, new lakes, and other changes; while the convulsions and disasters which took place in the West Indies, along the Pacific coast of South America, in the Sandwich Islands, in New Zealand, and other volcanic regions during the year 1868, must be fresh in the memory of every newspaper reader. The 164. The products of volcanoes, and the effects of volcanic action, have been sufficiently detailed in pars. 25 and 55. eruptions of Etna and Vesuvius are matters of everyday notoriety; the burying of Herculaneum and Pompeii, a subject of high historic interest. In 1783, the discharges of the Skaptar Jokul, in Iceland, continued for nearly three months, producing the most disastrous effects, as well as most extensive geological changes on the face of the island. "The immediate source, and the actual extent of these torrents of lava, have never been actually determined; but the stream that flowed down the channel of the Skaptar was about fifty miles in length, by twelve or fifteen in its greatest breadth. With regard to its thickness, it was variable, being as much as five hundred or six hundred feet in the narrow channels, but in the plains rarely more than one hundred, and often not exceeding ten feet." Again, the eruption of Mauna Loa (one of the Sandwich Island volcanoes), which took place in 1855, is described as progressing with amazing force and rapidity, and rolling its wide fiery floods over the mountain's summit down to its base with appalling fury. "Day after day," says an eyewitness, "the action increased, filling the air with smoke, which darkened our entire horizon, and desolating immense tracts, once clothed with waving forests, and adorned with tropical verdure. This eruption has now been in progress for nearly ten months, and still the awful furnace is in blast. The amount of matter disgorged is enormous: the main stream is nearly seventy miles long (including its windings), from one to five miles wide, and varying from ten to several hundred feet in depth." We quote these as instances of hundreds that might be advanced to show the extent of discharges from existing volcanoes. Whether as lava, pumice, scoriæ, dust, hot mud, or ashes, volcanic products, both on land and under the ocean, are materially adding to the structure of the rocky crust, just as in former epochs a similar function was performed by the granites, porphyries, basalts, traps, and trap-tuffs of the mineralogist. Nor is it to the mere accumulation of igneous rock-matter in certain localities that the student must look for the chief results of volcanic effort. As in former epochs, so even now we have lines and axes of volcanic elevation; and chains of hills, like those pointed out by Von Tschudi in Peru, and by Darwin in the Pacific, have arisen almost within the human era. RECAPITULATION. In the preceding chapter we have briefly indicated the nature and extent of the various accumulations that have taken place since the close of the Boulder-drift; in other words, since sea and land acquired the outlines of their present configuration, and were peopled by existing species. These accumulations we have classed under the head POST-TERTIARY, QUATERNARY, or RECENT, and subdivided into the following groups, according to the agents chiefly concerned in their aggregation: FLUVIATILE.-River accumulations and estuary deposits. MARINE.-Marine silts, sand-drift, shingle-beaches, &c. IGNEOUS. -Discharges of lava, &c.; earthquake displacements. As all these agencies are incessantly at work, some of the preceding accumulations are still in progress, others are comparatively recent, and some again of vast extent and antiquity. Indeed, when estuary deposits, alluvium in valleys, lake-silts, peatmosses, sand-drifts, coral-reefs, igneous discharges, upheavals and submergence of land are taken in the aggregate, they assume a geological importance not at all inferior, as far as amount is concerned, to any of the older formations. Paleontologically, they are also of considerable interest, affording evidence of certain general extinctions, as the mammoth, Irish deer, dinornis, dodo, solitaire, &c.; and of many local removals, as those of the elephant, rhinoceros, wild-boar, elk, bear, wolf, beaver, &c., from the surface of our own islands. In fact, the cosmical conditions of our planet forbid any cessation of progress; and thus, while its inorganic materials are being worn down, shifted, and reconstructed, its vitality must also undergo modifications, redistributions, and it may be extinctions. Theoretically, the accumulations of the present era are not only of high interest in themselves, but of prime importance, as furnishing a key to the complicated phenomena of former epochs: practically, they present many important features to the farmer, engineer, and navigator; and furnish us industrially with such substances as brick-clays, sand, marl, peat, pumice, pozzolana, sulphur, borax, petroleum, and other similar products. XVII. REVIEW OF THE STRATIFIED SYSTEMS.-GENERAL DEDUCTIONS. 165. To present a history of the structure and past conditions of our globe is the object of all geological inquiry. Astronomy may reveal its relations to the other orbs of the planetary system; Geology alone can unfold its individual constitution and structure. At the outset of his inquiry-on the very surface of the rocky crust he is about to examine the geologist is met by the fact, that everything beneath and around him is in ceaseless action, reaction, and change. The causes of this change he finds in the atmosphere that envelops the earth, in the waters that course and cover its surface, in the life that peoples it, in the chemical constitution of the substances of which it is composed, and in the fires that glow within its interior. These are ever and everywhere active; here wasting and degrading, there accumulating and reconstructing; here submerging the habitable dry land beneath the ocean, there upheaving the sea-bottom to form new islands and continents; and anon preserving in the re-formed material the remains of plants and animals as evidences of the world's geographical conditions at the time of their entombment. As at the present moment, so in all time past similar operations must have been going forward, and the results are manifested in the rock-formations of the solid crust which it is the province of Geology to investigate. 166. In this rocky crust we find sandstones that must have formerly spread out as sandy shores; conglomerates that formed pebbly beaches; shales that were the muds and clays of former lakes and estuaries; limestones that once were living coral-reefs; and coal-beds composed of the remains of a bygone vegetation. Here, also, we discover imbedded corals and shells and fishes that must have lived in the ocean; reptiles that thronged shallow |
