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. 75. 



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 " slicken-sides." 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 several minutes during each shock, afier 

 which they remained uninjured, and without any opening, al- 

 though the line of each crack was still visible. When all move- 

 ment had ceased, there were seen on the floor of the house, at 

 the bottom of each rent, small heaps of fine brickdust, evidently 

 produced by trituration. 



Faults.-— It is not uncommon to find the mass of rock, on one 



