3a 



KNOWLEDGE 



[February 1, 1892. 



with it. 'Sir. Powell, in bis popular narrative 

 of " Explorations of the Colorado Eiver," has 

 employed the term " base-level of erosion " to 

 express an idea which is of great importance in 

 jihysical geography. He was the first to give 

 it that definiteness which it formerly lacked. 

 The idea is this : when a smooth country lies 

 only a littb above sea-level, erosion takes place 

 at a merely nominal rate. The reason of this 

 is obvious, for the slopes being very slight, the 

 velocities, and therefore the transporting powers 

 of the streams are so feeble that they can do no 

 more than urge along the detritus brought down 

 from highlands round the margin of the country. 

 Soil formed on slopes or mounds of the expanse 

 is slowly carried off. The erosion is then said 

 to be at its " base-level," or nearly so. If any 

 given region of the earth remained for a long 

 time at the same height above the sea, it would 

 at last come to this state, and erosion would 

 practically cease. Many regions have done so 

 in the past, but the greater portion of the exist- 

 ing land of the globe has been subjected to 

 repeated up and down movements. Were it 

 not for such movements, the balance between 

 land and water could not be maintained. If 

 elevation took place at a rate faster than erosion 

 could keep pace with, the seas would become dry 

 land, and if subsidence went on everywhere 

 faster than deposition takes place in the seas, 

 the continents would disappear, and the globe 

 would be covered with water. Suppose any 

 region to have reached a base-level of erosion, 

 if it be depressed the sea spreads over it, and 

 it becomes an area of deposition. If, on the 

 other hand, it is elevated, new energy is im- 

 parted to the streams, and erosion takes place 

 more rapidly, because their slojies are increased, 

 and so their powers of corrosion and of trans- 

 portation become much greater. In this way 

 jiew topographical features are carved out and 

 long rapid slopes or clifl's are generated, and 

 we have seen that such features are vigorously 

 attacked on account of their height. During 

 the progress of the C4rand Canon, its anterior 

 spaces have for a time been at a " base-level of 

 erosion." Throughout the Quaternary, and most 

 of the Tertiary period, it has been rising ; and the elevation 

 varied from 11,000 to 18,000 feet, but the movements have 

 not been imiform. It appears to have alternated between 

 periods of activity and repose. One period of repose 

 jirobably occurred late in the Miocene, or early in the 

 I'liocene j^eriod. While it lasted the great Carboniferous 

 ] latform (in which the chasms occur) must have been 

 ]ilaned down to a very flat expanse, bounded of course by 

 tlie Terraces. But since then a general upheaval of several 

 thousand feet has taken place, giving a fresh impetus 

 to the river. 



Allusion may be made here to the volcanic phenomena 

 of the Toroweap Valley, which rims along a " fault," 

 whereby its western waU is made lower than the 

 eastern one. Above and beyond is the I'inkarit Plateau, 

 on the summit of which are a number of basaltic 

 cones in perfect preservation. " Very many wide and 

 deep floods of basaltic lava have poured over the edge of 

 the plateau into the lower Toroweap Valley, and upon the 

 great esplanade of the Canon more than 1500 feet below, 

 and spreading out into wide fields, have reached the brink 

 of the inner gorge. Pouring over its brink, the fiery 



-The M; 



Cauoii. 



cascades have shot down into the abyss, and pursued their 

 way many miles along the bed of the river. At one epoch 

 they had built up the bed of the Colorado about 400 feet, 

 but the river has scoured out its channel again and swept 

 them all away, regaining its old levels ; and is now cutting 

 the sandstones below. The spectacle of the lava floods 

 descending from the I'inkarit (plateau), as seen from 

 ' Vulcan's Throne," is most imposing." 



There are about 1'20 distinct cinder-cones on the 

 basaltic jJateau of the Uinkarit, and probably many more 

 volcanic vents. These often show a " linear arrangement " 

 as if they were so many vents occurring along the course 

 of a single fissure, but others, again, seem to be isolated 

 in this respect. In Fig. II. volcanic dykes are seen 

 (shaded dark). Then, the question arises whether they 

 are connected with " faults," as might seem natural 

 since " faults " are lines of weakness, up whicli steam and 

 lava might find their way. Captain Button says, if there is 

 any favoured direction taken by the volcanic forces, it would 

 seem to be along tlie upthrow of the "fault," and a few 

 miles from the line of dislocation. A truly wonderful 

 displacement forms the western boundary of the Uinkarit 



