252 



NATURE 



[yan. 1 6, 1879 



perished and undoubtedly they have shifted parts of their 

 courses somewhat ; but on the whole their tenacity of life is 

 wonderful, and the obstinacy with which they sometimes 

 maintain their positions is in powerful contrast with the 

 instabiUty of other topographical features. This charac- 

 teristic, however, fails at low levels. A river near its 

 mouth may often change its course ; but where the 

 country is high enough to enable it once to fasten its 

 grip it will hold it, despite all the changes to which the 

 surface of a continent is ordinarily subject throughout 

 the term of its secular existence. Its stability and per- 

 sistence will depend usually upon its altitude, or what 

 amounts to the same thing, upon the rapidity of its slope. 

 When that is small we may look for signs of inconstancy. 

 Other conditions might be formulated which could affect 

 it or modify it ; but on the whole the fact remains that 

 rivers have a remarkable power of maintaining their posi- 

 tions. It would be difficult to point out an instance where 

 a great river has ever existed under conditions more 

 favourable to longevity and stability of position than those 

 of the Colorado and its principal tributaries. Since the 

 epoch when it commenced to flow it has been situated in 

 a rising area. Its springs and rills have been among the 

 mountains and its slope has throughout its career been 

 continuously though slightly increasing. The relations 

 of its tributaries hare in this respect been the same, and 

 indeed the river and its tributaries have been a system and 

 not merely an aggregate, the latter dependent upon and 

 perfectly responsive to the physical conditions of the 

 former. And now we come to the point. The Colorado 

 and its tributaries run to-day just where they ran in the 

 Eocene period. Since that time mountains have risen 

 across their tracks, whose present summits mark less than 

 half their t otal uplifts ; the river has cleft them down to 

 their foundations. The Green River, passing the Pacific 

 Railway, enters the Uintas by the Flaming Gorge, and 

 after reaching the heart of this chain, turns eastward 

 parallel to its axis for thirty miles, and then southward, 

 cutting its way [out by the splendid caiion of Lodore. 

 Then following westward along the southern base of the 

 range for five miles, a strange caprice seizes it. Not 

 satisfied with the terrible gash it has inflicted upon this 

 noble chain, it darts at it viciously once more, and 

 entering it, cuts a great horse-shoe caiion more than 

 2,700 feet deep, and then emerging, goes on its way. 

 Thenceforward, through a tortuous course of more than 

 300 miles down stream the strata slowly rise — the river 

 almost constantly running against the gentle dip of the 

 beds, cutting through one after another, until its channel 

 is sunk deep in the carboniferous. Further down, near 

 the head of the Marble Caiion, the Kaibab rose up to 

 contest its passage, and a chasm more than 6,200 feet 

 in depth bears witness to the result. It is needless to 

 multiply instances. The entire province is a vast cate- 

 gory of instances of drainage channels running counter 

 to the structural slopes of the country. I am unable to 

 recall a single tributary to the right bank of the Colorado 

 which does not somewhere, and generally throughout the 

 greater part of its course, run against the dips. The 

 northern tributaries of the Grand Caiion have their entire 

 courses thus related. If we were to take the sums of the 

 lengths of the river and its right hand affluents, we should 

 find that at least three-fourths of that total length lay 

 where the streams run against the dips. 



It is clear, then, that the structural deformations of 

 the region — the faults, flexures, and swells, had nothing 

 to do with determining the present distribution of the 

 drainage. The rivers are where they are in spite of 

 them. As these irregularities rose up, the streams turned 

 neither to the right nor to the left, but cut their way 

 through them in the same old places. The process may 

 be illustrated by a feeble analogy with the saw mill. The 

 river is the saw, the strata are the timber which is fed 

 against it. The saw-log moves while the saw vibrates 



in its place. The river holds its position almost as 

 rigidly, and the rising strata are dissevered by its cease- 

 less wear. 



What, then, determined the situation of the present 

 drainage channels ? The answer is that they were deter- 

 mined by the configuration of the old Eocene lake-bottom 

 at the time the lake was drained. Then surely the water- 

 courses ran in conformity with the surface of the upper- 

 most Tertiary stratum. Soon afterwards that surface 

 began to be deformed by unequal displacement, but the 

 rivers had fastened themselves to their places and refused 

 to be diverted. This, then, is the key which unlocks for 

 the geologist the vestibule of the Plateau Country. The 

 rivers were born with the country itself, they are older 

 than its cliffs and cafions, older than its great erosion — 

 the oldest things in its Tertiary history ; nay, they are its 

 history, which we may yet read imperfectly in their caiion 

 walls. The mountains and plateaus are of subsequent 

 origin. They arose athwart the streams only to be cleft 

 asunder to give passage to the waters. The rivers amid 

 all changes have ever successfully maintained their right 

 of way. Such are the uses of the limited theorem of the 

 persistence of rivers.' I shall not attempt to suggest how 

 far it may be applicable to other regions, but I am con- 

 fident that any geologist visiting the Plateau Country 

 will be quickly overwhelmed with the conviction that it is 

 true there. 



In this connection it remains to add something to 

 indicate the magnitude of the work accomplished, and 

 the real extent of the obstacles which the Colorado has 

 accomplished in maintaining its existence. In the 

 Colorado itself, the maximum work has beep done at the 

 Grand Caiion (Fig. 2). This chasm is 2 1 7 miles in length, to 

 which should bs added properly the Marble Canon above, 

 69 miles long, since the two are continuous, and their 

 separation merely nominal. The average depth of the 

 Grand Caiion is a little more than 5,200 feet — almost 

 exactly one mile. Its maximum depth through the Kaibab 

 Plateau is nearly 6,300 feet, this depth being maintained 

 approximately as the river runs for about fifty miles. 

 Surely it might be thought that to cut such an abyss is 

 work enough in the life of one river however ancient of 

 days. But the summit of the Kaibab is Carboniferous 

 limestone. When the river began to run in this part the 

 whole local Mesozoic and lower Eocene series rested upon 

 the site of this plateau, but have since been swept away 

 together with a part of the Carboniferous rocks. The river 

 has cut through the entire fossiliferous system of strata 

 and now runs 2,000 feet deep in the archsean. The total 

 thickness of the fossiliferous system here is, or rather 

 was, very nearly 17,000 feet. Hence in its lifetime the 

 river has cut through about 19,000 feet of strata. 

 Through the remainder of the Grand Caiion the total 

 cutting has been from 2,000 to 3,000 feet less. As we 

 ascend the river the amount diminishes — not regularly 

 but with local maxima — until we approach the southern 

 base of the Uintas. The principal branch, the Green 

 River, has cut its channel into the quartzites of this 

 range even more deeply than the Colorado in the Kaibab. 

 Yet strangely enough the instant the Green is clear of the 

 mountains it enters a long stretch where the cutting has 

 been practically nothing. The explanation of this con- 

 trast will become obvious to the geologist by a mere 

 reference to the fact that where the cutting has been zero 

 the locality has been always at the base level of erosion, 

 and never above it. Only those parts which rise above 

 the base level are cut down. 



{To be continued.) 



' Mr. Jukes employed the same principle in explaining some features in 

 the lower courses of the rivers of Ireland. Quart, foutn. Geol. Soc. of 

 London, xviii. (1862), 37S, quoted in Jukes and Geikie's "Manual of 

 Geology," Third Edition, p. 454. [But the idea mav be found in Hutton's 

 great work the " Theory of the Earth," and in Play fair's " Illustrations. " 

 See particularly pp. 102 and 350 et seq. of the latter work. — Ed.] 



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