480 GEOLOGICAL EXCURSION TO THE ROCKY MOUNTAINS. 



Carboniferous 



.Jurassic (identified) DiiO 



. I nra -Trias 3,430 



Permian X5 1 



VII. Upper Aubrey limestone 805 



VI. Lower Aubrey limestone 1, 185 



[ V. Red Wall limestone 962 



Devonian Temple Butte limestone 



,..-,,, ( IV. Tonto (oaloareous and arenaceous shale) ) 



\ i I I 1 1 I ' I laill . - 



•i. io<; 



AJgonkiaa . - 



f 111. Tonto (sandstone) S 



( dinar (shales and limestones) 5. 120 ) 



II. Grand Canyon (sandstones with lava Hows in 



ill 



1,060 



upper part) (i. x:»o 



; 12,950 



Vishnu (bedded qnartzites and schists) l, OOOj 



26, 500 



GENERAL CONCLUSIONS. 



The interest afforded by the Grand canyon section is not restricted to 

 the mere petrographies! and stratigraphical diagnosis of its beds. It 

 would be (lilUciilt to find another locality where the geological changes 

 induced by faulting, folding, and volcanic eruptions can be so easily 

 observed at a single glance. As Capt. Dntton remarks: w ''Probably 

 there is no instance to be found in the world where an unconformity is re- 

 vealed upon such a magnificent scale, and certainly not amid such im- 

 pressive surroundings." If we attempt to read the pages of the gigantic 

 manual of geology, which is revealed to as from the brink of the great 

 chasm, we may decipher the following episodes in its ancient history: 



L. Energetic folding of the gneiss, and simultaneous or subsequent 

 intrusion of pegmatite dikes, which have also been folded. 



2. Complete erosion and planing down of the pre- Algonkian moun- 

 tains; deposition of 13,000 feet (-1,000 m.) of Algonkian sandstones and 

 shales. 



3. Eruption of diabases [the diabase dikes which cross the pegmatites 

 lie conformably between the Algonkian sandstones (whether surface 

 flows or intrusive sheets .') but do not penetrate the younger rocksj. 



•1. Upheaval and faulting of the Algonkian sediments and inclosed 

 eruptive beds. 



5. Transgression of the Upper Cambrian (Tonto) sandstones; incom- 

 plete erosion and planing off of the Algonkian land surface. [The 

 Algonkian beds are partially or totally wanting between the gneiss and 

 the Tonto series; in other words, the sediments thin out over the 

 ancient reefs of (he Cambrian sea.| 



0. The Silurian is wanting, and at the top of the irregularly dis- 

 tributed Devonian there is an unconformity by erosion (without dis- 

 cordance of stratification). These facts may be explained in either of 

 three ways: (I) The Canyon area became dryland before and after 

 Devonian time; (2) or, after deposition of the Silurian strata, they 



