originated. At certain places openings or craters occur (so called from their cup-like form-Gr. krater, a cup or bowl), and from these are ejected at intervals molten lava, fragments of rock, ashes, dust, hot mud, and various gaseous exhalations. Flowing from the crater over the surrounding country, the lava, after cooling, frequently forms thick layers of rocky matter, varying in compactness from hard basalt to open and porous pumice-stone. Ashes, dust, and volcanic mud accumulate in a similar manner, eruption after eruption adding to the height of the mountain, and ultimately giving to it a conical form. In this way have the cones of Etna, Vesuvius, and Hecla been formed, and in this way have eruptions modified the surrounding country, filling up valleys, creating crags and cliffs, enveloping fields, and burying cities, as in the case of Pompeii and Herculaneum. As with the active volcanoes of Europe, so with hundreds of others in various parts of the globe; and as these cones are now gradually rising and enlarging, so, looking at many of our older hills and mountainranges, we discover abundant proofs of a similar origin and mode of formation. As yet we have spoken of volcanoes as taking place only on land; but we have also evidence of their occurrence in the ocean, creating shoals and islands like many of those in the Pacific. In the one case, volcanoes are termed sub-aerial, in the other sub-aqueous. When taking place under water, the volcanic discharges of lava and ashes will be interstratified and mingled with the sedimentary matter of the ocean-an occurrence we shall afterwards find very common among the older rock-formations. Even when seated on land, they are generally in close proximity to the sea, and thus their ejections of lava, dust, and ashes falling into the water get overlaid by true sediments; and hence, also, the alternations of igneous and aqueous materials that occur so frequently in the crust of the globe. 26. Earthquakes, which are intimately associated with volcanoes, and but expressions of the same internal force, produce modifications of the earth's crust chiefly by fracture, subsidence, and elevation. During their convulsions the level plain may be thrown into abrupt heights, rent with chasms and ravines, or even be submerged beneath the ocean. Their general tendency is, therefore, like that of volcanoes, to diversify the surface of the globe, and to render irregular what aqueous agency is perpetually striving to render smooth and uniform. During violent convulsions, extensive alterations are sometimes produced on the face of a country; and of such we have frequent and abundant record within the historical era. Even within the present century, we know that a large tract at the mouth of the Indus was submerged, while a new district was raised from beneath the ocean; that the coast of Chili for many leagues was permanently elevated from six to ten feet; that a similar and more recent upheaval took place in North Island, New Zealand; and that in the West India islands, harbours have been sunk, towns destroyed, and rivers changed from their former courses. 27. The gradual Crust-Motions (whether elevations or depressions) connected with igneous agency are less obvious than the volcano and earthquake, but not on that account the less important or general. At present it is known, from repeated observation, that the northern shores of Scandinavia are gradually rising above the waters; the shores of Siberia, as well as of all the islands within the Arctic Circle, are fringed with numerous recent terraces; and large tracts along the eastern and south-western coasts of South America exhibit similar uprises. Such uprises, not being very perceptible, are apt to be under-estimated, or even disregarded; but when we cast our eye along the shores of our own island, and discover various ancient beaches or shore-lines stretching along above the present sea-level, at elevations varying from ten, twenty, forty, and sixty, to one hundred feet and upwards, we are then prepared to admit that such gradual uprises must be extensively modifying the appearance and conditions of the globe. Nor is it uprise alone, but depressions arising from the same cause are also observable in many regions, as among the islands of the Pacific, the Atlantic seaboard of the southern states of North America, the west coast of Greenland, and the southern shores of Norway. We know that the generic distribution of plants and animals is governed, in a great measure, by altitude above the sea; and one can readily perceive how such gradual uprises and depressions of the land must be gradually changing the character and distribution of the life upon its surface. Nor is it terrestrial existence alone that is influenced by such crust-movements: the sea-bottom is partaking of the same upward and downward motions, and marine life is even more sensitive than terrestrial to changes of depth and sea-bottom. RECAPITULATION. In the preceding chapter we have given a general outline of the causes now tending to modify the crust of the earth; that is, of the principal agencies concerned in the production of all geo logical change. These, we have said, were the Atmospheric, the Aqueous, the Organic, the Chemical, and the Igneous or Volcanic. By one or other of these agencies, or by a combination of them, are all the changes now taking place on the globe effected; and as we are warranted in concluding that these agents have similarly operated through all previous time, so to them must be ascribed the formation and structure of the solid crust. Rains, winds, and frosts must have always weathered and worn down; springs, streams, and rivers must have always cut for themselves channels, and transported the eroded material to lakes and seas, there to be spread out in layers and strata; and in these accumulations must the remains of plants and animals have been entombed, some swept from the land, and others buried as they lived in the waters. In this way, and by calling in the aid of chemical and organic agency to explain the occurrence of certain mineral-deposits and accumulations of vegetable and animal growth, we can account for the formation of all rocks which occur in layers or strata. On the other hand, as volcanic agency now breaks up the crust of the earth, elevating some portions and submerging others, and anon casting forth, from rents and craters, masses of molten matter, and showers of dust and ashes, so in former times must the same agency have fractured and contorted the solid strata, and cast forth molten matter, which, when cooled down, would form rockmasses, in which no layers or lines of deposit could appear. Besides modifying the earth's crust by upheaval and disruption, volcanic agency also produces a peculiar class of rocks; and these are found abundantly in all regions, from the recent lavas of Etna and Vesuvius to the basalts, greenstones, and granites of our older hills. We have thus, on and within the globe, a variety of agents ceaselessly active, and ceaselessly productive of change. The result of their operations is, and has ever been, the production of new rocks and new rock-arrangements; and these we shall now proceed to consider. III. GENERAL ARRANGEMENT, STRUCTURE, AND COMPOSITION OF THE MATERIALS CONSTITUTING THE CRUST OF THE GLOBE. 28. LAYING aside all speculations as to the interior constitution of the globe, concerning which we can know nothing by actual observation, we are warranted in saying, that the external portion or crust, accessible to human research, is composed of a variety of solid substances known as rocks. No matter whether in the form of soft and yielding clay, of loose sand and gravel, of beds of chalk and sandstone, or of masses of granite-all are termed by geologists, rocks and rock-formations. And the reason is obvious; the sand and gravel of the sea-shore are but the comminuted fragments of the cliffs above and have necessarily the same composition, while the mud and clay of the deeper waters are merely still finer comminutions of the same rocky material. Of such substances is the crust of the earth composed, and in one form or other we pass through them wherever we go beneath the surfacewhether tunnelling through the hills, or sinking coal-mines in the level valleys. How these rocks are arranged, and of what they are composed, are the subjects of our present inquiry. Stratified or Sedimentary Arrangement. 29. Judging from the operations of the modifying causes explained in the preceding chapter, one would naturally infer that all matter deposited as sediment from water would be arranged in layers along the bottom. Fine mud and clay readily arrange themselves in this manner, and sand and gravel are also spread out in layers or beds more or less regular. In course of time a series of beds will thus be formed, lying one above another in somewhat parallel order, thicker, it may be, at one place than at another, but still preserving a marked horizontality, and showing 30 IGNEOUS OR UNSTRATIFIED ARRANGEMENT. distinctly their lines of separation or deposit. Thus the miscellaneous debris (a convenient French term for all waste or worn material, wreck, or rubbish) borne down by a river will arrange itself in such layers along the bottom of a lake—the shingle and RIVER LAKE SAND Stratified Arrangement of Sediments. gravel falling first to the bottom, next the finer sand, and, lastly, the impalpable mud or clay, as represented in the preceding diagram. In course of time a series of layers will be formed, not perfectly parallel, one above another, like the leaves of a book, but still spread out in a flat or horizontal manner. One cannot look at the face of a quarry, or pass through a railway-cutting, without observing how very generally the rocks are arranged in beds and layers. These layers are technically known as strata (plural of stratum); hence all rocks arranged in layers-that is, arising from deposition or sediment in water—are termed aqueous, sedimentary, or stratified. In applying these terms the student will perceive that aqueous refers to the agency by which such rocks have been produced, sedimentary to the mode in which they have been formed, and stratified to the way in which they are arranged. Igneous or Unstratified Arrangement. 30. On the other hand, when we examine the rocky matter ejected from volcanoes, we observe no such lines of deposit, and no such horizontality of arrangement. In general, they break through the stratified rocks, or spread over them in mountainmasses of no determinate form-here appearing as walls, filling up rents and chasms, there rising up in huge conical hills-and in another region flowing irregularly over the surface in streams of lava. When such rocks are quarried or cut through, they do not present a succession of layers or strata, but appear in amorphous masses; that is, masses of no regular or determinate form (Gr. a, without, and morphe, form or shape). Thus, in connection with the stratified rocks, they present something like the annexed appearance,-A A A being stratified or sedimentary rocks lying |