366 TRANSACTIONS OF THE CANADIAN INSTITUTE. [VOL. V. 



Steps of the land valleys amounts to 1 50 feet per mile, and for the last 

 four miles, at the head of the amphitheatre even 600 feet per mile. 

 Similar declivities have also been pointed out among the tributary 

 amphitheatres of the Colorado River. The slopes of the drowned 

 valleys (as illustrated in figures 2 and 3), are thus seen to be a 

 succession of flat reaches, like base levels, and the margins of all are 

 characterized by steep slopes in all respects resembling the land valleys, 

 (figures 5 and 6) descending from high table-lands. The great stretches 

 of low gradients found amongst the drowned valleys, agree more or 

 less with the submerged plateaus, and appear to have been formed 

 when they were at base level of erosion, before their subsidence. 



If the Mississippi valley were elevated, the processes of canyon and 

 valley making would gradually dissect the margins of the newly 

 elevated table-land. Such appears to have been the case, at the time of 

 the formation of the now buried valley, discovered near New Orleans to 

 a depth of 1,000 feet. On the other hand, if the Mexican plateau were 

 reduced to base level, the valley above Atoyac would be a short fjord, 

 like that of the Gulf Cazonas (Figure 4)^ or like the buried valleys of 

 the lower Mississippi. 



If the analogy be complete enough, and numerous other phenomena 

 could be further cited in support of it, a most important geological 

 history can be interpreted from the submarine valleys. It has been 

 found that terrestrial movements are not uniform ; and although over 

 some continental regions the variation has been so slight as not to 

 disturb the course of the drainage, in others it has been affected. In 

 some cases barriers have been thrown across the valleys by the warp- 

 ing or tilting of the earth's crust ; such as that closing Lake Ontario 

 and other lakes and confined sea basins. The greatest amount of 

 terrestrial movement occurs in the mountain regions ; and the least 

 on the plains. There are also indications that the greatest rate of 

 depression takes place upon approaching the oceanic abysses. These 

 exaggerated movements named are usually parallel to mountain ranges, 

 and consequently most of them are transverse to the courses of the 

 submerged valleys, and consequently the terrestrial oscillations have 

 not materially affected their depths, though they may have somewhat 

 increased their slopes ; yet not to so great an amount as to prevent 

 the depths of the valleys dissecting the submarine plateaus (and pro- 

 ducing banks and islands) from being used as yard sticks for measuring 

 the extent of changes of level of land and sea. In the valleys parallel 

 to the mountain folds, it appears that the terrestrial movements have 

 given rise to deep basins. 



