522 Noble — Geology of the Grand Canyon, Arizona. 



phenomenon conditioned by the invasion of the limestones by 

 the diabase. It seens probable, as suggested by Diller (a, p. 

 72), that the serpentine which encloses the veins of asbestos is 

 derived from some mineral in the limestones and not from the 

 diabase. The limestones themselves are magnesian, and locally 

 siliceous in the form of chert bands and nodules. In another 

 part of the area the conversion of the shales to jaspers where 

 they are in contact with the diabase is evidence that the fuma- 

 rolic action accompanying the injection of diabasic magma 

 was manifested by aqueous and probably siliceous emanations 

 and was fairly intense. It seems possible that the operation 

 of the fumarolic action upon the elements already present in 

 the magnesian limestones might have been sufficient to convert 

 the more siliceous portions into serpentine. The occurrence 

 of the asbestos in veins that cut both the nodules of serpentine 

 and the limestones is evidence that the formation of the cross- 

 fiber asbestos was itself a somewhat later phenomenon. 



Age and Correlation. 



The Grand Canyon series is referred to the Algonkian in 

 the usage of the United States Geological Survey and is ten- 

 tatively correlated with the Keweenawan series of the Lake 

 Superior region and with the Llano series of Texas, following 

 the conclusions established by Walcott (&, p. 518). In the 

 Shinumo area the profound nature of the unconformity which 

 separates these strata from the basal Tonto sandstone is even 

 more striking than in the area described by Walcott, while 

 the certainty of their stratigraphic position is in its clearness 

 and spectacular character probably unparalleled in the world. 



Geologic History. 

 Outline. 

 The following sequence of events may be distinguished : 



1. Deposition of the Vishnu sediments, source unknown. 



2. Regional metamorphism. 



a. Subsidence and profound burial. 



b. Orographic movement of folding and compression 



resulting in recrystallization and schistosity, accom- 

 panied or preceded by injections of pegmatite. 



c. Elevation, accompanying or succeeding the orographic 



movement. 



3. Batholithic invasion of quartz diorite. 



4. Later pegmatitic injections. 



5. Long cycle of erosion carried through to a featureless pene- 



plain of no relief, which truncates the very roots of the 

 Vishnu structure. 



