Fluctuations in Level of the Quartzites. 135 



containing Scolithus linearis is found, and that a complete sub- 

 mergence would have been incompatible with such a deposit 

 The greater mass however, of what we are now acquainted 

 with was beneath the surface of the water ut some time dur- 

 ing that period, though probably not at its commencement, for 

 it is highly probable that a slow snbeidence was in progress 

 during this time, and this may have been but a continuance of 

 the depression that constituted the change from the elevated 

 position just described to the present one. 



That the shores were precipitous is a matter of observation. 

 That the water along shore, was of moderate depth, is inferred 

 from the nature of the deposit. The following .section will 

 render this more clear : 



a, Quartzite. b b, Potsdam sandstone, showing , beach structure and iucludedi' 

 quartzite fragments, d, Potsdam sandstone, c, Conglomorate. 



This is an ideal section, but no feature is introduced that 

 has not been verified by personal observation, and no vio- 

 lence has been done to nature in the combination. It is here 

 used as a convenient substitute for the seven natural sections 

 accompaning the original presentation. I beg leave also to 

 refer to the rive natural sections accompanying the paper of 

 Prof Eaton on the relations of these rocks, to be found on 

 page 125 of this volume. The sandstone formed at this lime 

 is marked b b. Near its junction with the quartzite, well 

 defined cross lamination occurs. This is beautifully showu 

 near Ableman. Near the vertical junction there also occur,, 

 imbedded in the sandstone, large and chiefly angular masses 



