14 Essay introductory to Geology. 



found to be beut upwards or downwards, thus indicating in which direction 

 their prolongations are to be sought. 



§ 3. Besides these dislocations, we often find the continuity of series of 

 horizontal strata interrupted by valleys, which, as it were, effect breaches 

 in them, having the same series, layer for layer, repeated on the opposite 

 sides of the valley. In such cases we often find tiie base or outlet of the 

 valley overspread with broken fragments of the strata, wiiich we conceive 

 must have origiually extended continuously across it. Now to put an 

 analogous case ; if we were to observe an abrupt interruption in a regular 

 wall, built in orderly courses of masonry, and were to find this breach 

 loaded by shattered fragments of the materials wliich were deficient in 

 their regular place ; could we then hesitate to attribute the said breach 

 to violence, which had affected the wall subsequently to its original 

 erection ? And will not the same reasoning apply with equal force to the 

 valley in question r The rocky fragments which are spread over its base, 

 are generally found to be rounded into boulders and pebbles, exactly like 

 those which are worn by the action of water. Now if we were to attribute 

 the excavation of these valleys to the action of violent currents of water, 

 we might seem to be running into theory; but it is scarcely possible to 

 describe the phenomena without employing language which must naturally 

 suggest such a conclus'on. These valleys are called valleys of excavation; 

 they traverse the horizontal strata exactly in the same manner that the 

 graver of the artist, in cutting an intaglio on a stratified onyx, penetrates 

 its various layers. The terminal edges of the masses of our strata, appear 

 to have been very generally abraded by the same action which has exca- 

 vated these vallies, and thus are made to end with steep declivities or 

 escarpments, as they are called. The plains at the foot of the escarpment, 

 are extensively overspread with rounded fragments of the abraded rocks ; 

 and this gravel generally contains remains of the elephant, hippopotamus, 

 rhinoceros, hyaena, &c. 



