Geology. 145 



II. Geology. 



1. Geology of the Navajo Country — a Reconnaissance of Parts 

 of Arizona, New Mexico, and Utah; by Herbert E. Gregory. 

 United States Geological Survey, Professional Paper 93. Pp. 

 161, pis. 34, figs. 3. Washington, 1907. — This publication is a 

 welcome contribution on one of the little known parts of the 

 United States. It is of interest to geologist and geographer, and 

 deals with subjects of economic as well as scientific interest. 

 The work might be termed missionary geology, for it was done 

 chiefly for the benefit of the Navajo Indians. The report eon- 

 tains the usual wealth of detail, but the character which gives it 

 unusual value is the straightforward manner in which debatable 

 questions are discussed. Whether the author is right or not there 

 is no doubt as to what he thinks. 



Light is shed on several questions of unusual interest con- 

 nected with the wonderland of Grand Canyon and the Plateau 

 Country. Especially important are the contributions to the 

 physiography of the Triassic and Jurassic periods. The oldest 

 Triassic formation of the region, the Shinarump conglomerate, is 

 described as a stream-laid deposit of sand and gravel spread out 

 over a graded plain of great extent. The younger Triassic rocks 

 containing the petrified forests were eroded to some extent and 

 covered in Jurassic time with vast deposits of sand. The Win- 

 gate and Navajo sandstones are regarded as equivalent to the 

 Vermilion Cliff (Triassic of former writers) and White Cliff 

 sandstones of the plateau region, which have been famous since 

 they were first described for a peculiar type of crossbedding and 

 for marvelous landscape architecture. These sandstones possess 

 characteristics supposed to indicate desert conditions. The 

 question of origin of the sand which in Utah reaches thicknesses 

 of about 3,000 feet was raised nearly half a century ago. It is 

 now known that the sandstone occupies large parts of four States 

 and that its general character is the same in all places. Thus 

 the interesting possibility is presented of a great Sahara in 

 western America in mid-Mesozoie time. 



In the Navajo country the Wingate and Navajo sandstones 

 are separated by thin beds of shaly limestone and correlated with 

 the similarly constituted La Plata group. The correlation with 

 La Plata on the one hand and with the Vermilion- White Cliff 

 group on the other raises again a much debated question in 

 correlation. 



Many splendidly developed physical features are described and 

 illustrated, such as the intrenched meanders in San Juan Canyon, 

 remarkable natural bridges, and natural monuments of unusual 

 expression. A commendable departure is an attempt to establish 

 the correct spelling and origin of unusual names, such as Moen- 



Am. Jour. Sci.— Fourth Series, Vol. XLV, No. 266.— February, 1918. 

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