Februaky 1, 1892.] 



KNOWLEDGE 



31 



plateau. This is the great Hurricane " fault." Its full 

 length is not known, but this enormous fracture extends 

 at least 40 miles. In the southern Wiill of the Grand 

 Caiion it appears with a displacement of about 1500 feet, 

 throwing down the whole country to the west of it, and 

 making a great cliff. The effect is as if a table were split 

 across the middle and one side lifted up a foot or more. 

 In one place the displacement is estimated at 12,000 feet ; 

 this is near the Valley of the Virgin, the Eocene strata 

 on one side actually standing nearly 1000 feet lower than 

 the much older (and therefore lower) Carboniferous rocks 

 on the other side. Of the date at which this movement 

 took place we shall speak later on. 



The later geological history of the region of the Colorado 

 plateau may now be briefly summed up. All the time when 

 the great denudation was going on, by which the Mesozoic 

 rocks overlying the platform of the Caiions were denuded 

 and the Terraces formed, it may be concluded that the 

 entire region occupied a level not much above the sea. 

 It was at a " base-level of erosion." The rivers and 

 streams no longer corroded their channels, but meandered 

 slowly along the Carboniferous platform, dragging their 

 burdens of soil along with them. This state of things was 

 reached about the close of the Miocene period, and it 



Fig. II. — Dykes in the "Wall of tlie Grand Caiiou. 



lasted a considerable time. Then a new epoch of upheaval 

 set in, and the work of cutting the Grand Canon began. 

 By this upheaval the country was hoisted up from 2000 to 

 3000 feet. Meanwhile a change of climate had taken 

 place, the supplies of aqueous vapour failed, and tlie 

 present dry time began, the rainfall being very slight. 

 So most of the old streams dried up. Such streams as 

 remained alive corroded their channels, but most of the 

 platform suflered no more destruction than it does now by 

 the slow waste that goes on. With this upheaval the 

 Hurricane " fault " was developed, although it is possible 

 that some slight displacement existed there already. If 

 so, its magnitude was increased. At the same time the 

 earliest volcanic outbursts took place. At length the 

 uplifting paused for a time. Then the volcanoes ceased to 

 work. The river for a time worked vigorously at the 

 strata below it, the upward movement of the rocks having 

 greatly increased both its corroding and transporting 

 powers. But it quickly cut down until it found a new 

 " base-level of erosion." Then corrosion ceased. During 

 this second period of comparative repose the work of 

 erosion was confined to the sapping of the newly-formed 

 cliffs of the Cailon. The chffs then began to recede away 

 from the river ; thus gradually was the broad avenue of the 

 outer chasm made. When the cliffs of this 

 outer chasm had receded two or three miles 

 away from the river, another and more active 

 period of upheaval set in. Once more the 

 forces of elevation were brought into play, and 

 this time the country was hoisted up more 

 than before. Again the river began to corrode, 

 or deepen, its channel. The displacements 

 were increased in magnitude until the present 

 " throw " of the " faults " was attamed. These 

 earth-movements affected the equilibrium 

 between the internal and external forces at 

 w^ork on the so-called crust of the earth. The 

 volcanoes became active again and poured out 

 their lava streams. It was this second uplift 

 that gave to the river the power to deepen its 

 channel until it assumed its present condition. 

 At present the elevating force, whatever it may 

 be (and this is a question not yet settled), is 

 inactive. Does it depend partly, as some think, 

 on denudation ? For it is conceivable that the 

 removal of a considerable thickness of rocks 

 from any given continental surface, may so 

 diminish the downward pressure, due to their 

 weight acting on the lower parts of the earth, 

 as to disturb the equilibrium, and so force on 

 an upward movement ; but this is only a specu- 

 lation. Anyhow, it seems that earth-movements 

 and volcanic action are in some way connected 

 together, for it is only during upheaval that 

 volcanoes are active, and they are never associ- 

 ated with regions where depression is going on. 

 The (ictirc volcanic regions of the globe at the 

 present day are all regions of recent elevation, 

 as may be seen by consulting a physical atlas. 



This second upheaval was greater than 

 the one which preceded it, amounting pro- 

 bably to 3000 or 4O0O feet. The epoch at 

 which it took place is no doubt a recent 

 one, geologically. It probably began near 

 the close of the Pliocene period. '' At present 



' The Tertiary or Cainozoic era is divided by geologists 

 into tliree periods. The oldest of these is the Eocene, 

 then follow the iliocene and Pliocene. 



