Jan. 23, 1879] 



NATURE 



273 



been investigated as thoroughly as circumstances and my this is of the simplest order. -; If we were to see a fault 

 very limited qualifications would admit. Its original cutting a particular stratum we should know that the 

 attraction consisted in the enormous displays of volcanic stratum was older than the fault. By parity of reasoning 

 energy there in former times, to whidi I cannot here we know that the outer canon is older than the faults 

 venture to allude any further. The structure of the district because they cut its trough and dislocate its floor trans- 

 is also otherwise very interesting, and has been worked out I versely. If the faults were older the river would have 

 with much care and patience, and in great minute- 

 ness of detail. It will be possible at present to 

 give nothing more than a categorical statement of 

 certain results. To master the evidence would re- 

 quire the handling of a large amoimt of detail, and 

 unwarrantably protract discussion. 



The structure of these plateaux is identical with 

 what Prof Powell has described as Kaibab struc- 

 ture, being in fact a northward continuation of the 

 same belt which he has described and delineated 

 in his well-known section of the Grand Cafion which 

 cuts across this series of displacements at a right 

 angle. The faults which have blocked out the 

 plateaux and intervening valleys are of prodigious 

 length, and the amounts of dislocation are ver>- great 

 — greater in the high plateaux than in the Kaibabs. 

 The age of these displacements is an important 

 landmark in the historj' of the country, and that 

 age can be fixed with confidence as late Pliocene, 

 and continuing into the Ouartemary, and probably 

 down to the present time. 



With this fact in our possession as a datum we 

 come now to the history of the caiions. The Grand 

 Cafion first makes its appearance in the epoch of 

 the faults. It suddenly bursts into view as a less 

 than half-formed thing, with walls ranging from 

 2,000 to 2,700 feet high, late in Pliocene times. 

 But it presents itself under somewhat unexpected 

 circumstances, for it had been in the condition in 

 which we first find it for a considerable period. 

 The work of vertical erosion had long been sus- 

 pended, the channels had ceased to grow deeper, 

 and the energies of the river had for an unknown 

 period been employed in another kind of occupation 

 to which rivers have been frequently known to be- 

 take themselves under certain common conditions. 

 It was widening its caiion and making a flood plain 

 in which to meander. This any river will inevitably 

 do when it has sunk its channel to the limiting 

 depth which local circumstances prescribe for it. 

 When that limit is reached it will attack its own 

 banks whether they be v.alls of rock or nothing but 

 gravel and loess, and will thereafter meander or 

 squirm from side to side. There are numerous 

 places along the Upper Colorado and its tributaries 

 where this is abundantly exemplified. From local 

 causes the fall of the river has for a space been 

 diminished, the flow has been sluggish, sediment 

 has been deposited, the river has ceased to erode 

 its bottom, it has attacked its walls, and the canon 

 has been widened. 



If now the reader will look at the section of the 

 Grand Canon (Fig. 3) he will perceive that it is a 

 canon within a caiion. The walls are in two leaps 

 with an intermediate terrace. The upper or outer 

 canon is usually from three to six miles wide, and 

 the inner cafion meanders within the upper, some- 

 times close to one upper wall, sometimes to the 

 other, but usually with a middle terrace on both 

 sides. The inner and the outer caiion represent 

 two periods, the outer one of course being formed 

 first — formed no doubt originally as a narrow 

 gorge — which was widened while the river was unable 

 to ciit vertically. The middle terrace is the final flood 

 plane of the old caiion. Arid now the faults come to 

 our assistance in determining the two periods. The 

 outer caiion is older than the faults; the inner one is 

 coeval with them. The reasoning by which we determine 



Fig. 4.-^Pa^riHra^W«e^C&S>B, Virgin K;ver, a trtb*»arjr<rf-Ae-Gc*jrado. 



planed an even grade across them regardless of the dips 

 of the strata just as it is doing to-day 3,000 feet below. 

 As it is— if the side gorges wotild permit us to travel along 

 the middle terrace— we should be compelled every time 

 we crossed a fault to clamber up or down its face. 

 Thus, then, as we draw near the close of the Pliocene 



