GEOLOGICAL SOCIETY. \\ 



either side of which, throughout its course, a continued 

 series of contorted stratified masses, rolling and heaving 

 like the weaves of a stormy ocean, enibarras the pro- 

 gress of geological investigation. 



It has been long ago noticed, that the deposition of 

 the grauwache and transition strata, appears to have 

 been subjected to much interruption ; that the order of 

 their succession is extremely variable ; and that while 

 in some situations, certain deposits were accumulated in 

 great thickness, they were at other points wholly 

 absent. 



The Lewistown argillo-siliceous beds, which are dis- 

 tinguished by such a remarkable succession of fossil 

 marine plants, furnish an example in accordance with 

 these views.* Occupying the bottom of a deep trough, 

 between two lofty ridges which are formed of a differ- 

 ent material and incline at opposite angles, these beds 

 broken and distorted by violent action and apparently 

 unconformable to the subjacent rocks, bear evidence of 

 a more recent origin. f Towards the eastern termina- 

 tion of this trough, they are seen rising up at a high 

 angle and then folded back against the mountain ; ap- 

 pear to abut upon the siliceous rocks of the ridge ; dip- 

 ping about twenty degrees to the north west. J In the 

 centre of the narrows they dip at an angle of thirty de- 

 grees in the same direction towards the ridge,§> and at 

 the upper or western entrance the inclination changes 

 to a very gentle slope from the mountain. || 



* PL iv. Fig. 6. 

 § Pi' iii. Fig. 9. 



t PI. iv. Fig. 7. 

 I! PI. iii. Fig. 10. 



t PI. iii. Fig. 8. 



