The Shinum-o Area. 523 



6. Sudden incoming of the shallow Unkar sea. 



7. Deposition of the Unkar sediments and succeeding strata of 



the Grand Canyon series in shallow water (or upon land ?) 



8. Intrusion of diabase sills. . 



9. Orographic movement of block faulting and tilting accom- 



panied or succeeded by elevation. 



10. Long cycle of erosion carried through to a peneplained sur- 



face of small relief. 



11. Incoming of the Tonto sea, succeeded by the deposition of 



the Paleozoic strata of the wall of the Grand Canyon. 



The earliest event which is decipherable in the geologic 

 history of the Shinumo area is found in the blurred and 

 mangled record of the Vishnu schists. Far back in the dark 

 ages of geologic time a thick series of more or less arkose 

 sands and muds was accumulating upon a subsiding floor. 

 So much may be reasonably inferred from the mineralogical 

 character of the quartz schists of the mica and hornblende 

 type. So dim and vague is the record that the base of this 

 series, the floor upon which it was laid down, the thickness, 

 and the location of the land mass from which it was derived 

 must, perhaps, remain forever unknown. Following the long 

 accumulation and burial of these sediments came an orographic 

 movement which wrote across the older manuscript in a newer 

 and bolder hand, blurring the ancient writing with the stamp 

 of deep-seated regional metamorphism, and imparting to the 

 manuscript the aspect of a palimpsest. The regional meta- 

 morphism is conceived to have been brought about by deep 

 burial of the sediments, followed by folding and compression, 

 which engraved upon them the characters of recrystallization 

 and schistosity in slow process of time, and accompanied by 

 their elevation into lofty mountains. Somewhat later the cores 

 of these mountains were intruded by great masses of igneous 

 rock, here in the form of quartz diorite, followed by peg- 

 matitic injections. Perhaps while this orographic movement 

 was still in progress the forces of erosion were already at work. 

 Then followed a tremendous cycle of erosion carried through 

 to the very end, planing away the ancient mountains to the 

 basal roots, and reducing hard and soft rocks alike to an utterly 

 flat and monotonous level. Such was the completion of the 

 cycle of this vast unknown and unnamed aeon of time. 



The next event is the beginning of another great cycle of 

 sedimentation resulting in the deposition of the Grand Canyon 

 series of Unkar and Chuar strata, ushered in by the sudden 

 invasion of a shallow sea which swept over the featureless 

 desert surface of the Yishnu plain, depositing the basal conglom- 

 erate of the Unkar. The clue to the inferences as to the 

 character of this incoming sea, of the rock mantle which it 



