Ch. v.] 



FISSURES IN STEATA. 



61 



could not be expressed. From the point of view here selected, the under- 

 lying beds of the perpendicular schist, a, are visible at h through a small 

 opening in the fractured beds of the covering of red sandstone, d c?, while 

 on the vertical face of the old schist at a' a" a conspicuous ripple-mark 

 is displayed. 



It often happens that in the interval between the deposition of two sets 

 of unconformable strata, the inferior rock has not only been denuded, but 

 drilled by perforating shells. Thus, for example, at Autreppe and Gusigny, 

 near Mons, beds of an ancient (primary or paleozoic) limestone, highly 



Fig. 84. 



Jxmction of unconformable strata near Mons, in Belgium. 



iiichned, and often bent, are covered with horizontal strata of greenish 

 and whitish marls of the Cretaceous formation. The lowest and there- 

 fore the oldest bed of the horizontal series is usually the sand and con- 

 glomerate, a, in which are rounded fragments of stone, from an inch to 

 two feet in diameter. These fragments have often adhering shells at- 

 tached to them, and have been bored by perforating mollusca. The 

 solid surface of the inferior limestone has also been bored, so as to ex- 

 hibit cylindrical and pear-shaped cavities, as at c, the work of saxicavous 

 mollusca ; and many rents, as at 6, 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 and faults. — N'umerous rents may often be 

 seen in rocks which appear to have been 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 frag- 

 ments of stone, evidently derived from the fracture of the contiguous 

 rocks. 



It is not uncommon to find the mass of rock, on one side of a fissure, 

 thrown up above or down below the mass with which it was once in 

 contact on the other side. This mode of displacement is called a shift, 

 slip, or fault. " The miner," says Playfair, describing a fault, " is often 

 perplexed, in his subterraneous journey, by a derangement in the strata, 

 which changes at once all those lines and bearings which had hitherto 

 directed his course. When his mine reaches a certain plane, which is 

 sometimes perpendicular, as in A B, fig. 85, sometimes oblique to the 

 horizon (as in C D, ibid.), he finds the beds of rock broken asunder, 

 those on the one side of the plane having changed their place, by sliding 

 in a particular direction along the face of the others. In this motion 

 they have sometimes preserved their parallelism, as in fig. 85, so that 



