320 Prof. Nicholson — Valleys of Erosion. 



the other, whilst the bed of the stream is often the uninterrupted 

 surface of a single stratum. There is, therefore, no reason to call in 

 the aid of any mechanical disturbance to account for the phenomena 

 of Watkins Glen. There is, however, a marked physical peculiarity 

 in the strata of the Glen upon which many of its features can be 

 shown to depend. The beds, namely, whilst nearly horizontal, are 

 intersected in the most regular manner by two sets of vertical joints, 

 the one set striking N. and S., whilst the others run E. and W. 

 These two sets of joints are, therefore, perpendicular to one another, 

 and they divide the entire mass of strata into a series of oblong 

 blocks. The N. and S. joints are placed at intervals of about three 

 feet from one another ; the E. and W. joints at a distance of five or 

 six feet, and the faces of both are approximately vertical. Other 

 irregular joints are not absent, but these may be left out of account. 

 As I have before remarked, the stream runs nearly due east and 

 west, so that its course corresponds with the one set of joints, and 

 is perpendicular to the other. The effect of these joints on the work 

 of erosion is readily seen. All the cascades over which the stream is 

 precipitated are formed by the faces of the north and south series of 

 joints, the stream gradually cutting back from the face of one joint 

 to that of the next. On the other hand, the bed of the torrent be- 

 tween the cascades has been scooped out along the east and west 

 series of joints. The rock itself is so uniform in character, and so 

 devoid of alternations of harder and softer beds, that there can be no 

 hesitation in regarding the above as the true explanation of the for- 

 mation of this wonderful ravine. 



Very different causes have been at work in the formation of the 

 well-known gorge of the Genesee at Eochester, and the still more 

 celebrated ravine through which the Niagara Kiver cuts its way 

 below the Great Falls. In both these instances we have examples of 

 gorges which owe their origin to the action of running water upon 

 beds of unequal hardness. At Eochester, the Genesee Eiver lias 

 made for itself a long and precipitous ravine, which is terminated at 

 the city of Eochester itself by the beautiful Falls of the Genesee. 

 The strata through which the river cuts have a slight southerly dip, 

 and the stream itself flows nearly due north, so that its direction 

 is at right angles to the inclination of the beds. As seen immedi- 

 ately below the Falls, the sides of the gorge are seen to be com- 

 posed of a very hard, compact, bluish or grey limestone (the Niagara 

 Limestone), capped by a mass of genuine glacial drift. On reaching 

 the bottom of the ravine, however, the Niagara Limestone is seen to 

 be underlaid by bluish-grey fissile shales (the Niagara Shales), re- 

 plete with Wenlock fossils. The bed of the stream below the Falls 

 is excavated in these comparatively soft shales ; and it is over the 

 much more resisting limestone that the river is precipitated to form 

 the cataract. The action of the stream under these circumstances 

 is extremely obvious, and has been repeatedly pointed out in the 

 case of the Great Falls of Niagara. The stream, namely, gradually 

 cuts its way back, by constantly wearing away the soft shales 

 below the Fall, and thus leaving the hard limestone which forms 



