1880. 



87 



[Stevenson. 



chasms, the hasalt appearing sometimes on one side, sometimes on the 

 other, and occasionally on both sides. One may easily determine whether 

 or not the canon is wholly beyond the limits of the ancient gorge, since if 

 it be, the wall is sheer to the top ; whereas, if the course be along the old 

 channel-way, and the absence of the basalt be due to erosion, the bench 

 previously covered by the lava is still distinct. The cross sections exhibit 

 the features very satisfactorily ; thus in passing from one canon to the 

 other at a little way above the junction, one would find the condition 

 shown in Fig. 3. 



Fig. 2.— Section across canons of Canadian and Mora rivers near tlieir junction. 



The basalt covers the level bench between the two canons, while the 

 higher wall of each is composed from top to bottom of the Dakota rocks. 

 But at a mile and a half further up the streams the structure is as shown 

 in Fig. 3 where the basalt appears on both sides of each canon. Here both 



Fig. 3.— Section across cafions of Canadian and Mora rivers, two miles above 



their junction. 



streams have cut their way through the basalt, whereas in the other, both 

 canons are outside of the sheet. 



This new channel-way has been digged out of rocks of unequal hardness, 

 and the corrasion seems to have gone on with equal rapidity everywhere. 

 The gorge is as deep w^here the basalt plate appears in both walls as it is 

 w^here the basalt is on one side or wholly away from the caiion. The only 

 observable difference is that, where cut through the basalt, the caiion is 

 very narrow, and the cutting has been confined to the immediate channel- 

 way ; whereas, the gorge is wide and shows a "bottom" where it has 

 been worn in the sedimentary rocks only. The Canadian and the Mora 

 are exceedingly rapid and very imperfectly loaded. Their corrasive power 

 is far greater, therefore, than might be supposed, if their volunae alone 

 were considered. 



