Jan. 16, 1879] 



NATURE 



251 



a period of stupendous erosion. The uplifting however 

 was unequal. The comparatively even floor of the old lake 

 was deformed by broad gentle swells rising a little higher 

 than the general platform. In consequence of their 

 greater altitude these upswellings at once became the 

 objects of special attention from the denuding agents and 

 were wasted more rapidly than lower regions around 

 them. Here were formed centres or short axes from 

 which erosion proceeded radially outwards, and the strata, 

 rising very gently towards them from all directions, were 

 bevelled off. As the erosion progressed so also did the 

 uplifting of these local centres or axes, thus maintaining 

 the maximum erosion at the same localities. It is a 

 most significant fact that the brunt of erosion is directed 

 against the edges of the strata and not against their 

 surfaces, provided the stratification is but little disturbed. 

 Usually such an uplift will have one diameter longer 

 than another, and we may call the greater the major-axis. 

 The strata dissolve away in all directions by the waste 

 of their edges, and after the lapse of long periods the 

 newest or uppermost strata will be found encircling the 

 centre of erosion at a great distance — the next group 

 below will encircle it a little nearer, and so on. 



This has been the history of each of the sub-divisions 

 of the Plateau Country. Upon the western and northern 

 sides of the Colorado five of these centres are now easily 

 discerned. By far the largest and probably the oldest is 

 around the Grand Caiion. All these had their inception 

 in Miocene time, though the one around the Grand Cafion 

 may go back into the upper Eocene. The district known 

 as the San Rafael Swell is by far the best suited for study. 



If we stand upon the eastern verge of the Wasatch 

 Plateau and look eastward we shall behold one of those 

 sublime spectacles which fill even the calmest observer 

 with awe and amazement. From an altitude of more 

 than 1 1,000 feet the eye can sweep a semicircle with a 

 radius of nearly seventy miles. It is not the wonder 

 inspired by great mountains, for only two or three peaks 

 of the Henry Mountains are well in view, and these with 

 their noble alpine forms seem as strangely out of place 

 as Westminster Abbey would be among the ruins of 

 Thebes. Nor is it the broad expanse of cheerful plains 

 stretching their mottled surfaces beyond the visible 

 horizon. It is a picture of desolation and decay ; of a 

 land dead and rotten with dissolution apparent all over its 

 face. It consists of a series of terraces all inclining 

 upwards towards the east. We stand upon the lower 

 Tertiary rocks and right beneath our feet is a precipice 

 leaping down across the edges of the level strata upon a 

 terrace 1,200 feet below. This cliff stretches away north- 

 ward gradually swinging eastward, and finally southward, 

 describing a rude semicircle around a centre about forty 

 milesto the eastward. At the foot of this cliff is a terrace 

 about six miles wide of upper Cretaceous beds inclining 

 upwards towards the east very slightly, and at that 

 distance it is cut off by a second great cliff plunging 

 down 1,800 feet upon middle Cretaceous beds. This 

 second cliff describes a smaller semicircle concentric 

 with the first. From the foot of the second cliff the 

 strata again rise through a width of about ten miles and 

 are cut off again by a third series of cUffs as before. 

 There are five of these concentric lines of cliffs. In 

 the centre there is an elliptical area forty miles long and 

 twelve to twenty wide, its major axis being north and 

 south, which is as completely girt about by rocky walls 

 as the valley of Rasselas, but such walls as Dr. Johnson 

 never dreamed of. We have given it the name of the 

 Red Amphitheatre. Yet, if we look back to Eocene time, 

 we shall find that the whole stratigraphic series, up to 

 the Eocene inclusive, covered this amphitheatre. Nearly 

 10,000 feet have now gone, and the floor is near, or 

 quite, at the summit of the Carboniferous rocks. At pre- 

 sent the Amphitheatre is drained by two streams which 

 cut across it and find their way, one into the Green, 



the other into the Colorado, below the junction of the 

 Grand. 



Still more vast is the erosion which has taken place 

 from the vicinity of the Grand Cafion. Here the Car- 

 boniferous strata form now the floor of the country, though 

 a few patches of Trias still remain in the vicinity of the 

 river. But the main body of the Triassic rocks stands, 

 now fifty miles north of the river, and beyond them, in a 

 series of great terraces, rise the Jurassic, Cretaceous, and 

 Tertiary formations— the latter capped with immense 

 bodies of volcanic rock. The greater part of the erosicm 

 was accomplished in Miocene time. 



It will be seen that these local uplifts are important in 

 determining the subdivisions of the area and the distri- 

 bution of the maxima and minima of degradation. We 

 may see here a correspondence which is worthy of close 

 attention. Those areas which have been uplifted most 

 have been most denuded. I have asked myself a hundred 

 times whether we might not turn this statement roimd, 

 and say that those regions which have suffered the greatest 

 amount of denudation have been elevated most, thereby 

 assuming the removal of the strata as a cause and the 

 uplifting as the effect ; whether the removal of such a 

 mighty load as ten thousand fett of strata from an area 

 of ten thousand square miles may not have disturbed the 

 earth's equilibrium of figure, and that the earth, behaving 

 ; as a {^«ai-/-plastic body, has reasserted its equilibrium by 

 I making good a great part of the loss by drawing upon its 

 I whole mass beneath. Few geologists question that great 

 masses of sedimentary deposits displace the earth beneath 

 them and subside. Surely the inverse aspect of the 

 problem is a priori equally palpable. That some such 

 process as this has operated in the Plateau Coimtry 

 looks at least very plausible, and, if there could be found 

 independent reasons for believing in its adequacy, the 

 facts certainly bear it out. Yet its application is iiot 

 without some difficulties, and the explanation is not quite 

 complete. Granting the principle, it will be still diffi- 

 cult to explain how these local uplifts were inaugurated ; 

 and we can only refer them to the agency of that myste- 

 rious plutonic force which seems to have been always 

 at work, and whose operations constitute the darkest 

 and most momentous problem of dynamical geology. 

 On the whole it seems to me that we are almost driven 

 to appeal to this mysterious agency to at least inaugurate, 

 and perhaps in part to perpetuate, the upward movement, 

 but that we must also recognise the co-operation of that 

 tendency which indubitably exists within the earth to 

 maintain the statical equilibrium of its levels. The only 

 question is, whether that tendency is merely potential or 

 becomes partly kinetic; and this again turns upon the 

 rigidity of the earth. But it is easy to believe that, where 

 the masses involved are so vast as those which have been 

 stripped from the San Rafael Swell, and from the 

 Kaibabs around the Grand Caiion, the rigidity of the 

 earth may become a vanishing quantity. 



Let us turn now to a law which forms a most important 

 link in the chain of discussion — a law without a thorough 

 comprehension of which the structural geologist in the 

 Plateau Country would see very little except Sphinxes, 

 but one which, when he has fully saturated his mind 

 with it, will enable him to translate many mysteries. This 

 law may be called the persistence of rivers. It is a very 

 simple one, but its uses are wonderful ; indeed those who 

 have found it so invaluable in the Plateau Country often 

 wonder why so little use has Lean made of it elsewhere. 

 If the study of this region should accomplish nothing 

 more than drawing this principle from its modest retire- 

 ment and installing it in its rightful place in the logic of 

 geology it will still have accomplished a great result. 

 But the law has its limits, which we cannot overstep with 

 safety. 



Of all the changing features of a continent the least 

 changeful are its great rivers. Undoubtedly rivers have 



