Disturbances in Clay Slate. 



293 



In the latter case the flexure is so great that the slate is partially 

 broken ; and this is the case frequently ; showing that when the dis- 

 turbance took place, the rock was only imperfectly plastic. The 

 quarry where these flexures are exhibited, has been so much exca- 

 vated, as to leave a wall 20 or 30 feet high ; and excepting at these 

 cross seams, the laminae are remarkably even ; so that the phenom- 

 enon is rendered very striking. It must obviously have resulted 

 from the unequal action of some disturbing force — perhaps that by 

 which the strata were elevated — whereby one portion of the rock 

 was pressed forward, faster than the rest ; though in some places not 

 enough to separate, but only to bend, the slate while in a soft con- 

 dition. 



At the Gorge, or Glen, in Leyden, I found a series of such slides 

 on a small scale, exhibited by the slate ; as in the following sketch ; 

 though perhaps the rock ought to be regarded as mica slate. (No. 

 416.) Here the echellon movement took place in a direction at right 

 angles to that described above. 



Another disturbance, much more remarkable, appears at the quarry 

 in Guilford above referred to. In the following sketch the observer 

 faces the north, and looks directly upon the edges of slate, as it re- 

 mains at the north end of the quarry, in its natural ^position. The 

 almost uniform dip of the strata, in every part of this quarry, is near- 

 ly 90° ; leaning, however, a little to the east. And such is their po- 

 sition at the north end of the quarry, to the height of 15 feet, as rep- 

 resented in the sketch. But from 10 to 15 feet of the upper part of 

 the slate are bent towards the west, so as to incline to the horizon at 

 almost every angle, from 0° to 90°. Where the flexure commences, 

 the laminae of the slate are quite broken off, and not simply bent, as 

 in the case of the disturbance in a perpendicular direction above de- 

 scribed. Hence I infer that the former flexure was not produced so 

 soon as the latter ; not indeed until the rock had become perfectly 

 consolidated. The quarry lies upon the western slope of a hill of 



