96 Feather A tonhaugh s Geological Report, 



Washington and Georgetown are immediately underlaid by 

 the gneiss, and in the environs of this last place, especially 

 along the line of Rock creek and on the canal, evidences are 

 already perceived in the alternate southeast and northwest dip 

 of the rocks, of that extensive anticlinal movement by which 

 all the rocks along the entire line of the Potomac have been 

 affected, as high up as the great bituminous coalfield. In the 

 various localities here, where the rocks are exposed, the true 

 dip of the rocks is so contradicted by the cleavage, that great 

 and patient attention is required to distinguish between that 

 and the stratification ; but there are some instances on the canal, 

 on leaving Georgetown, where the southeast dip of the gneiss 

 is sufficiently clear; after some distance the stratabecome fissile, 

 with large veins of quartz, are elevated into an almost vertical 

 position, and then dip to the northwest, at a very high angle, 

 as far as the great falls. On approaching the falls, the bed of 

 the river presents a singular spectacle : sharp, isolated masses 

 of dark, glossy, micaceous slate, turned upon their edges, lie 

 bare for a great distance, and occupy a large area, resembling 

 the breakers of a boisterous sea suddenly petrified. But the 

 river has long ago abandoned this part of its bed, whilst the 

 proof of the rocks having been reduced to their present state 

 by its former action, is abundant in the immense quantity of 

 pot-holes in the rocks, some of them two or three feet in 

 diameter, occasioned by the whirling motion of fragments of 

 quartz or other hard mineral matter, in depressed parts of the 

 slate, which, when continued a long time, make very deep 

 holes, as may be seen in the beds of all rivers where the water 

 is low and runs quick. This perforating process is one of the 

 causes of the destruction of strata upon a large scale, the holes 

 becoming so deep and numerous that at length the floods have 

 strength enough to disintegrate the strata, and subsequently 

 break up the fragments. In long periods of time, water has 

 power to wear its way through the stoutest mineral masses, 

 and 1 know of no place which affords a better study of the 



