AEGONKIAN ROCKS OP (iE VGRAND CANYON @@K 
she, COLORADO), 
CONTENTS. 
Introduction. 
Geographic position and distribution. 
Nomenclature. 
Stratigraphic relations. 
Chuar terrane. 
Unkar terrane. 
Vishnu terrane. 
Sediments and conditions of deposition. 
Geologic age. 
Correlation. 
INTRODUCTION. 
Tue Algonkian rocks of the Grand Canyon are unique among 
the known unconformable pre-Cambrian rocks both of America | 
and of Europe. Nowhere else has the geologist an equal oppor- 
tunity to study such a series of ancient sediments nearly as they. 
were laid down on the bed of the Algonkian sea. At no other 
known locality are there such extended and complete exposures 
of all the beds forming a great series of pre-Cambrian strata, 
permitting of such certainty in the determination of stratigraphic 
position and succession. 
The first recorded notice of these rocks is that by Major 
J. W. Powell in the account of his famous explorations of the 
Colorado River of the West and its tributaries. He says in 
his report of 1875, page 212, that above the granites there are 
beds of hard, vitreous sandstone, of many colors, that add but . 
little more than 500 feet to the height of the walls, and yet, 
owing to their nonconformity with the overlying Carboniferous 
rocks, they are 10,000 feet in thickness. On page 213 we learn 
that the rocks unconformably beneath the sandstone series are 
composed of metamorphosed sandstones and shales which have 
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