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CHARLES R. KEYES 



valley. While in general the rocks are only gently deformed they are 

 locally sharply folded, highly inclined, or even standing on edge. As 

 has been recently shown' bolson plains, of which the Estancia Plains 

 are a type, have not the simple substructure that is commonly ascribed 

 to them. They do not appear to be necessarily any newer or later 

 than the plateau plains which overlook them. Their constructional 

 detrital covering is no more important than that of the plateau plains. 

 The wash-deposits brought down from the mountainous periphery are 

 relatively of small importance. 



On the other hand, it has been clearly demonstrated in a number 

 of cases that the bedrock surface of the bolson plains, or that plane 

 beneath the detrital covering, is a planation-surface worn out on the 



Fig. 9. — Alternation of Cretaceous shales and sandstones at Hagen, thickness 

 about 4,000 feet. 



beveled edges of the indurated sedimentaries. This feature is par- 

 ticularly well displayed in the plains between the Ortiz and Sandia 

 mountains. At the Una de Gato, near Hagan, the structure is as 

 represented below (Fig. 9). The dotted line represents the plains 

 surface; it extends to the right a distance of 20 miles. 



The character of this beveling of highly inCHned beds, in this case 

 Cretacic sandstones, is admirably shown in the accompanying view 

 near Los Cerrillos, from a photograph taken by Dr. D. W. Johnson. 

 The horizontal beds at the top and above the old planed surface are 

 composed of volcanic breccias, which in turn are overlain by late 

 mesa clays and sands. (See Fig. 12.) 



Characteristic rock-masses. — There are represented in the Estancia 

 region five principal kinds of rock-masses. In the great fault-scarp 



I American Journal of Science (4), Vol. XV, p. 207, 1903. 



