structure; but at a little distance regular columns appear, at first horizontal; after extending thus a few feet they curve gradually upward and at twenty feet distance stand nearly vertical. The columns are from four to six sided, thirty feet long, and from one to four feet in diameter. The effects of heat upon the conglomerate are well marked. The melted basalt appears to have flowed over a small ridge of conglomerate in a stream fifty feet thick, and the forms and position of the columns were determined by the varying surface.* 171. The basaltic scenery of New South Wales delineated by Professor Dana, rivals that of the Scottish Isles. Some of the basaltic ridges rise to the height of four thousand feet above the level of the sea; and fissures or caves occur similar to those of Staffa. Figure 86 presents a view of the basaltic pillars at Kiama point. The sea enters through the channel seen in the cut, which is twenty feet broad and *Prof. Dana, Geology of U. S. Exploring Expedition. eighteen high; advancing by a subterranean passage for a distance of two hundred feet, it strikes against a wall of basaltic columns with deafening roar, and rises through an orifice to the height of one hundred feet. This is called a blow-hole or spout-hole. Kiama Blow-hole in Basaltic Columns. 172 Numerous examples of trap rocks and basaltic pillars occur in the United States-as the "Palisades" of the Hudson River, Titan's Piazza on Mount Holyoke, East and West Rocks near New Haven, Connecticut, &c. Vertical dikes with horizontal pillars in Maine and North Carolina present such regularity of figure and position as to have induced the belief that they were products of human skill. Trap rocks are associated with the metaliferous strata of Lake Superior and California, and successive rows of basaltic columns, constituting mountain masses occur in the vicinity of the Columbia River, in Oregon. 173. The columnar and globular structure so characteristic of volcanic rocks, especially of basalt, early engaged the attention of Geologists and induced Mr. Gregory Watt to institute experiments in the year A. D. 1804 to ascertain its origin. He melted seven hundred pounds of basaltic rock, in a reverberatory furnace. He was enabled to effect its fusion with a less degree of heat than would have been required by an equal weight of pig-iron. When melted, it appeared as a dark, liquid, tenacious glass; a portion of it taken out and allowed to cool rapidly, retained its vitreous appearance, resembling obsidian. The remainder was left in the furnace, and occupied eight days in cooling; it was then cold externally, but still retained a considerable degree of internal heat. The interior of the mass, which had cooled slowly, exhibited a great number of spheres, which as they enlarged pressed laterally against each other, until they were cemented into regular prisms, with segments of alternate convex and concave surfaces. The articular structure and regular forms of basaltic columns seem therefore to have resulted from the crystalline arrangement of the particles while the rock slowly cooled under pressure. 174. The Feldspathic varieties of trap rocks, as trachyte, constitute the central or axial portions of mountains, while the exterior portions are composed of basalt; for instance, the summit of Mount Loa is clinkstone, while all its sides are basalt. This has been attributed to the difference in fusibility and specific gravity of the constituents, feldspar and augite. The temperature at which feldspar solidifies is sufficient to keep augite quite fluid. The augite also by gradual cooling crystallizes and is converted into hornblende, for these two minerals differ only in crystallization, and that is determined by temperature and rate of cooling. VOLCANIC ROCKS. 175. The term lava is applied to any rock which has flowed from a volcanic vent, but it is frequently restricted to the melted rocks erupted by active volcanoes or those recently extinct. The constituents of such lava are the same as those of the trap rocks, and we have feldspathic lava, and augitic lava, according to the predominance of either of those ingredients; but we do not find the compactness and crystalline structure so frequently exhibited, as in the d older igneous rocks. Rapidity of cooling and difference of temperature and pressure, adequately account for the diversity of forms which igneous rocks present, while their constituents are essentially the same. 176. Lava bursting through the sides of volcanic cones presents itself in dikes similar to trap dikes; columnar structure also is exhibited by them. When thoroughly melted lava cools rapidly, it resembles glass and is called volcanic glass, or obsidian; it is usually of a dark smoky color. Its fracture presents bright surfaces and sharp edges; it has been used by the Mexicans for mirrors, and knives. Lava of less glassy structure, and of a pitch-like luster is called pitchstone. The cellular slaggy matter ejected from volcanoes is called scoria; when it is composed of feldspar, it is porous and fibrous and is then called pumice. It is so light as to float upon water. Scoria and pumice are the frothy scum upon the surface of melted lava, and become very abundant when hot lava comes in contact with water. Minute par |