. Elements of geology. Geology. PART I. CHAPTER VIII. 113 Columnar and Globular Structure. although in these it is rarely exhibited in such regular polygonal forms. It has been already stated that basaltic columns are often di- vided by cross joints. Sometimes each segment, instead of an angular, assumes a spheroidal form, so that a pillar is made up of a pile of balls, usually flattened, as in the Cheese-grotto at Ber- trich-Baden, in the Eifel, near the Moselle. (Fig. 102.) The Fig. basaltic pillars of the Kdsegrotte, BertrichBaden, halfway between Treves and Coblentz. Height of grotto
. Elements of geology. Geology. PART I. CHAPTER VIII. 113 Columnar and Globular Structure. although in these it is rarely exhibited in such regular polygonal forms. It has been already stated that basaltic columns are often di- vided by cross joints. Sometimes each segment, instead of an angular, assumes a spheroidal form, so that a pillar is made up of a pile of balls, usually flattened, as in the Cheese-grotto at Ber- trich-Baden, in the Eifel, near the Moselle. (Fig. 102.) The Fig. basaltic pillars of the Kdsegrotte, BertrichBaden, halfway between Treves and Coblentz. Height of grotto from 7 to 8 feet. basalt, there, is part of a small stream of lava, from 30 to 40 feet thick, which has proceeded from one of several volcanic cra- ters, still extant, on the neighbouring heights. The position of the lava bordering the river in this valley, might be represented by a section like that already given (Fig. 100. p. 112.), if we merely suppose inclined strata of slate and the argillaceous sand- stone called greywacke to be substituted for gneiss. In some masses of decomposing greenstone, basalt, and other trap rocks, the globular structure is so conspicuous that the rock has the appearance of a heap of large cannon-balls. A striking example of this structure occurs in a resinous tra- chyte or pitchstone-porphyry in one of the Ponza islands, which rise from the Mediterranean, off the coast of Terracina and Gaieta. The globes vary from a few inches to three feet in di- ameter, and are of an ellipsoidal form. (See Fig. 103.) The whole rock is in a state of decomposition, " and when the balls," says Mr. Scrope, " have been exposed a short time to the wea- ther, they scale off. at a touch into numerous concentric coats, like those of a bulbous root, inclosing a compact nucleus. The. Please note that these images are extracted from scanned page images that may have been digitally enhanced for readability - coloration and appearance of these illustrations m
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Keywords: ., bookauthorlyellcharlessir17, bookcentury1800, booksubjectgeology
