^74 



NATURE 



\7aii. 23, 1879 



period only the outer canon was completed. When was it 

 •commenced ? At present we cannot give an answer, though 

 I hope we may soon be able to do so ; but I should be sur- 

 prised to find its commencement dating beyond Pliocene 

 *ime. The best conjecture which I am at present able 

 to frame would place the birth of the Grand Cafion since 

 the middle of that epoch. The commencement of the 

 cutting of the inner canon was contemporary with the 

 commencement of a new uplifting of the Kaibabs— an 

 4iplifting which extended as far north as the Wasatch, 

 and southward to unknown regions in Arizona, through a 

 >belt having its maximum width just where the Grand 

 Canon crosses it. The amount of this uplifting was very 

 variable, ranging from 2,000 to 4,000 feet. It was during 

 this period of elevation that the faulting took place. The 

 level of the river's bed was at once disturbed ; its old 

 •energies were reawakened, and its ancient labours 

 resumed. From that epoch to the present day the 

 :river grinding like " the mills of God " has slowly but 

 resistlessly sunk itself to solemn depths in the earth. 



Wonderful and impressive as are the great canons, 

 they are no more so than some other features. Chief 

 -among the objects of special interest is the vast array of 

 colossal cliffs, which stretch across the country with 

 seemingly interminable length in a grouping which is half 

 order half disorder. The number is indeed very great, 

 their altitudes generally impressive — 1,500 feet being 

 •common, and 2,000 feet not very uncommon, while the 

 -distant riew of cliflTrising above cliff, one beyond another, 

 yet seeming to be united, is often seen. Each strati- 

 graphical series has a series of cliffs, planned, sculptured, 

 and painted in a style peculiarly its own, and the several 

 styles differ, as decidedly and constantly as human archi- 

 tecture among distinct races of men. These distin- 

 guishing characters developed under one homogeneous 

 process can be traced to the lithological composition 

 and texture of the rocks which are powerfully contrasted 

 between any two series. The constancy of result in any 

 ^iven series may also be traced to the constancy with 

 which that series preserves one set of characters over a 

 ^reat extent of country. I may be mistaken — perhaps 

 ■from the circumstances it is more apparent than real — 

 but I imagine no region in the world hitherto explored 

 exhibits rocks where the texture and lithological characters 

 are so strongly pronounced, so strongly contrasted among 

 themselves, and yet where there is so little horizontal 

 variation in the characters of each group over vast areas. 



In the Plateau Country we have to do with an arid 

 region, and the aridity tends to reduce the amount of 

 disintegration. On the other hand it is a lofty country 

 giving a rapid descent to all its waterways, and their 

 transporting power is of a very high order — the rocks are 

 swept bare of debris and kept naked to the attacks of 

 the elements. This tends powerfully to quicken the dis- 

 integration. The number of inches of annual rain is 

 Jess than one-sixth the number in the Mississippi valley, 

 but every inch in the plateaux may do sextuple work. 

 Probably, however, the rate in the plateaus is on the 

 whole slower, but the disproportion is much less than 

 might have been anticipated if we had considered the 

 rainfall alone. 



To comprehend the oi'igin and perpetuation of cliffs it 

 is necessary to expand these general relations into some 

 ■detail. I have stated that the attack of erosion is directed 

 against the edges of the strata and but slightly against 

 the horizontal surfaces. These surfaces being but little in- 

 clined, water has but little energy, as it courses over them, 

 either to erode or to transport. But in the myriads of 

 gulches the steepness of their sides enables the water 

 fto keep the edges of the strata naked, and the water is 

 assisted powerfully by the aridity of the climate and the 

 absence of vegetation. Now when the edges of a thick 

 •series of vertically heterogeneous strata are exposed, there 

 will always be some stratum softer or more readily disin- 



tegrated than the others. The elements attack it, and 

 soon a long under-cut is formed, and the rocks above 

 robbed of a part of their support, cleave off vertically, 

 and a great slab falls in ruins. The fallen fragments 

 and rubble form a talus, but being now in a com- 

 minuted state, they become a much easier prey to dis- 

 solution than when in the solid wall, and they gradually 

 moulder away. All that is necessary is that the talus 

 should dissolve fast enough to keep the perishable 

 stratum exposed to attack, and this is almost uni- 

 versally the case. The great cliffs are massive beds of 

 sandstone and limestone, resting upon perishable cal- 

 careous and gypsiferous shales. The rapidity with which 

 the cliff wastes away and recedes by erosion is measured 

 by the power of resistance to weathering in the shales 

 below and not by the massive beds on its face. By 

 further analysing the details of erosive action, we have 

 no difficulty in explaining the origin and causes of the 

 different styles of architecture, the sculpture of the repeti- 

 tive forms, and all their train of phenomena, both normal 

 and abnormal. 



And now a few words about the cause of cafions. This 

 problem has been so admirably and satisfactorily solved 

 by Messrs. Powell and Gilbert that I have no better 

 excuse for saying anything about it than a desire to fill 

 what would otherwise be a serious gap in the discussion. 



The fall of the Colorado through the caiions is between 

 seven and eight feet per mile — nearly twenty times as 

 great as that of the Ohio and Mississippi and nearly 

 seven times as great as that of the Missouri below the 

 Yellowstone. It is a fierce torrent — a series of quickly- 

 recurring rapids. Its lateral gorges have usually a greater 

 descent. The tools with which the river works are sand 

 and gravel held in suspension by the water, hurled along 

 at race-horse speed, and scouring like a sand-blast 

 machine the naked rocks of its bed. But there is one 

 thing more, and it is a crucial point. The Platte has 

 about the same fall through the plains as the Colorado 

 through the canons ; it has its sources High up in the same 

 mountains j it flows through a desert ;lit carries a huge 

 load of sand, but from Denver to Plattesmouth has not 

 the semblance of a canon. The trouble with the Platte 

 is that it carries too mtich sand. A river of given volume 

 and velocity can carry in suspension only a definite load 

 of sediment of given coarseness. When that limit is ex- 

 ceeded the excess will be precipitated upon the bottom 

 protecting it from the scour of the gritty particles which 

 are carried in suspension. But if the supply of sand be 

 not in excess of the power of the current to keep it in 

 suspension, none will be deposited except locally, and 

 the bed-rock will experience the full attrition of the sand- 

 blast. The Platte is the case of an overloaded stream 

 while the Colorado is slightly underloaded and in a condi- 

 tion to produce the maximum erosion. 



The study of the Plateau Country has during the last 

 nine years been the work of the Survey under Prof. J. W. 

 Powell. Comparatively Uttle has been published about it 

 because it has been felt by him that until the subject 

 could be presented in systematic and thoroughly intel- 

 ligible form it would be a mistake to accumulate frag- 

 mentary literature and encumber a splendid subject with 

 a chaos of unconnected observations. But the work 

 approaches completion and has developed into form in 

 the minds of the workers, and it is hoped that the results 

 will soon be before the world. If the geology of the 

 Plateau Country shall therein be set forth in a manner 

 commensurate with its importance, and full justice done 

 to the revelations it affords, I believe that physical 

 geology will have received important additions. I cannot 

 close without paying a just tribute to Prof. Powell, the 

 director of their work. His direction of the Survey has 

 not been limited to the perfunctory duties of an admi- 

 nistrative officer. On the contrary he has furnished those 

 whom he has called to his assistance with methods of 



