. Elements of geology. Geology. PART L CHAPTER V. 75 Fissures in Strata. example, at Autreppe and Gusigny, near Mons, beds of ancient stone commonly called transition limestone, highly inclined, and often bent, are covered with horizontal strata of greenish and whitish marls of the cretaceous formation, which will be men- tioned in a future chapter. The lowest and therefore the oldest bed of the horizontal series is usually the sand and conglome- rate, a, in which are rounded fragments of stone, from an inch Fig. Junction of unconformable strata near Mons, in Belgium. to two feet in diame
. Elements of geology. Geology. PART L CHAPTER V. 75 Fissures in Strata. example, at Autreppe and Gusigny, near Mons, beds of ancient stone commonly called transition limestone, highly inclined, and often bent, are covered with horizontal strata of greenish and whitish marls of the cretaceous formation, which will be men- tioned in a future chapter. The lowest and therefore the oldest bed of the horizontal series is usually the sand and conglome- rate, a, in which are rounded fragments of stone, from an inch Fig. Junction of unconformable strata near Mons, in Belgium. to two feet in diameter. These fragments have often adhering shells attached to them, and have been bored by perforating mol- lusca. The solid surface of the inferior limestone has also been bored, so as to exhibit cylindrical and pear-shaped cavities, as at c, the work of saxicavous moUusca; and many rents, as at b, which descend several feet or yards into the limestone, have been filled with sand and shells, similar to those in the stratum a. Fractures of the strata.—Numerous rents may often be seen in rocks which appear to have beeif simply^broken, the separated parts remaining in the same places; but we often find a fissure, several inches or yards wide, intervening between the disunited portions. These fissures are usually filled with fine earth and sand, or with angular fragments of stone, evidently derived from the fracture of the contiguous rocks. The face of each wall of the fissure is often beautifully pol- ished, as if glazed, striated, or scored with parallel furrows and ridges, such as would be produced by the continued rubbing to- gether of surfaces of unequal hardness. Those polished surfaces are called by miners " ; It is supposed that the lines of the strise indicate the direction in which the rocks were moved. During one of the late minor earthquakes in Chili, the brick walls of a building were rent vertically in several places, and made to vibrate for s
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