1880.] 85 [Stevenson. 



ly three miles. How far it went below the month of the Mora caiion was not 

 ascertained. The continuity of the flow is distinct throughout and the 

 general features of the rock in the Canadian canon are the same with those 

 observed in the crater itself. 



A solid crust forms quickly on the surface of cooling basalt. It is prob- 

 able, therefore, that the Mora river was not long displaced ; but that it, as 

 well as the Canadian, soon regained its channel and began once more the 

 work of corrasion. The features shown in the canons are of no small in- 

 terest, for the extent of erosion prior to the basalt flow is as clearly exhib- 

 ited as is the extent of the erosion since that event. The features are best 

 studied in the Mora canon, at the mouth of which the following measure- 

 ments were obtained : i 



1. From river surface to bottom of basalt 230' 



2. " same to top of basalt 630' 



3. " " brink of the chasm 890' 



4. " " top of wall 1090' 



The chasm is therefore 230 feet deeper than when the basalt flood took 

 possession of it. 



As the present caiions do not wholly coincide with the ancient gorges, the 

 numerous curves exhibit the base of the basalt sheet very satisfactorily. The 

 old valley of the Mora was benched or sfightly terraced, so that the lava rests 

 against a wall of sandstone, and overflows the bench of which that is the 

 escarpment, as is shown in fig. 1 : 



Fig.l. 



Fig. 1.— Showing form of the ancient caiion. a. — Daliota sandstone. 6.— Basalt. 



A breccia frequently appears in the bottom of the sheet, as though the 

 molten rock had caught up debris in its flow. 



The basalt terrace at thejunction of the canons indicates that the ancient 

 canon of the Canadian was much wider than that in which the river now 

 flows. 



The south-west wall of the old canon lies at nearly a mile further from 

 the river than does the present wall, which is capped by the sheet of basalt. 

 It is veiy evident that before the basalt outpouring occurred the caiions had 

 been digged down at their junction to 860 feet below the present level of the 

 plain, and that the depth was reduced to 470 feet by the influx. But since 

 the streams regained their channel-ways, material to the depth of 620 feet 

 has been removed by corrasion ; so that the vertical cutting since the flow 

 ceased is very nearly equal to that done before the flow began. This cut- 

 ting is wholly the I'esult of corrasion, the wear of the streams upon their 

 beds. 



