Noble — Geology of the Grand Canyon, Arizona. 497 



Art. XLIY. — Contributions to the Geology of the Grand 

 Canyon, Arizona. — The Geology of the Shinumo Area 

 (continued) ; by L. F. Noble. (With Plate V.) 



Part II. 



General Geology {continued). 



Algonkian — Grand Canyon Series. 

 Name. 



Distribution in the Grand Canyon. 

 Stratigraphic Position, Structure, and Distribution in the 



Shinumo Area. 

 Sediments of the Unkar Group. 

 Preliminary Outline. 

 Detailed Section. 



Comparison with the Type Section in Unkar Valley. 

 Diabase Intrusive in the Unkar. 

 Occurrence. 

 Petrography. 

 Variations in character. 

 Contact Metamorphism. 

 Conclusions. 

 Age and Correlation. 

 Geologic History. 

 Bibliography. 



Algonkian. 

 Grand Canyon Series. 



Name. — The unaltered pre-Cambrian sedimentary rocks of 

 the Grand Canyon region were first studied by Walcott 

 (Walcott, b) at the eastern end of the Kaibab division of the 

 Canyon, They are described as a series of sedimentary rocks, 

 12,000 feet in thickness ; comprising limestones, shales, sand- 

 stones, and interbeclded flows of lava ; separated both from 

 the underlying Vishnu schists and from the overlying Cam- 

 brian sediments by profound unconformities ; and exposed over 

 a considerable area in the greater depths of the Grand Canyon 

 and in the inter-canyon valleys of the north side. To this 

 series of sedimentary rocks the name " Grand Canyon 

 series" was given by Walcott. A slight unconformity of 

 erosion was found to occur in the middle of the series. The 

 strata lying below this minor unconformity were designated 

 as the "Unkar terrane," while those lying above it were desig- 

 nated as the " Chuar terrane." The Unkar terrane derives its 

 name from Unkar valley, in which these strata are typically 

 exposed. The Chuar terrane is named from its typical 

 exposures in Chuar valley. These two valleys are parallel 

 inter-canyon valleys of the north side of the Colorado river 

 in the area described by Walcott. 



Distribution in the Grand Canyon. — There are six locali- 

 ties within the Grand Canyon between the mouth of the Little 



Am. Jour. Sci.— Fourth Series, Vol. XXIX, No. 174.— June, 1910. 



