. Elements of geology. Geology. PART 1. CHAPTER VIII. Ill Columnar Structure of Volcanic Rocks. but they have most commonly from five to seven sides. They are often divided transversely, at nearly equal distances, like the joints in a vertebral column, as in the Giant's Causeway, in Ire- land. They vary exceedingly in respect to length and diameter. Dr. MacCulloch mentions some in Sky which are about 400 feet long; others, in Morven, not exceeding an inch. In regard to diameter, those of Ailsa measure nine feet, and those of Morven an inch or less.* They are usually straight, but sometimes cur
. Elements of geology. Geology. PART 1. CHAPTER VIII. Ill Columnar Structure of Volcanic Rocks. but they have most commonly from five to seven sides. They are often divided transversely, at nearly equal distances, like the joints in a vertebral column, as in the Giant's Causeway, in Ire- land. They vary exceedingly in respect to length and diameter. Dr. MacCulloch mentions some in Sky which are about 400 feet long; others, in Morven, not exceeding an inch. In regard to diameter, those of Ailsa measure nine feet, and those of Morven an inch or less.* They are usually straight, but sometimes curved ; and examples of both these occur in the island of Staffa. In a horizontal bed or sheet of trap the columns are vertical; in a vertical dike they are horizontal. Among other examples of the last-mentioned phenomenon is the mass of basalt, called the Chimney, in St. Helena (See Fig. 98.), a pile of hexagonal prisms, 64 feet high, evidently the remainder of a narrow dike, the walls of rock which the dike originally traversed having been removed down to the level of the sea. In Fig. 99. a small portion of this dike is represented on a less reduced It being assumed that columnar trap has consolidated from a fluid state, the prisms are said to be al- ways at right angles to the cooling surfaces. If these surfaces, there- voicanic dike composed of horizontal fore, instead of being either perpen- prisms. St. Helena. dicular or horizontal, are curved, the columns ought to be inclined at every angle to the horizon ; and there is a beautiful ex- emplification of this phenomenon in one of _ ^ the valleys of the Vivarais, a mountainous \J ^ ' district in the South of France, where, in the ^ "^^-N i^idst of a region of gneiss, a geologist cn- —^ counters unexpectedly several volcanic cones Small portion of t7ie dike in of loosc sand and scorice. From the crater of one of these cones, called La Coupe d'Ayzac, a stream of lava descends and occupies the bottom of a narrow v
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Keywords: ., bookauthorlyellcharlessir17, bookcentury1800, booksubjectgeology
