Campbell — Fractured Bowlders in Conglomerate. 231 



Art. XXI. — Fractured Bowlders in Conglomerate /* by 

 Makius R. Campbell. 



During a visit to the Deer Creek coal field of Arizona, 

 which the writer made in the autumn of 1903, countless num- 

 bers of fractured bowlders were found on the outcrop of a 

 coarse conglomerate, which seemed to require unusual and 

 peculiar surroundings to account for their present conditions. 

 Accordingly some hasty notes were taken of field relations 

 and a few typical specimens secured for further study. A 

 group of these fractured bowlders is shown in figure 2 and 

 the following descriptions may throw some light on their 

 mode of origin. 



The region in which the phenomenon was observed is an 

 irregular syncline of Cretaceous and Carboniferous rocks in 

 Pinal County, about ten miles east of Dudleyville at the 

 junction of the San Pedro and Gila rivers, better known per- 

 haps as the Deer Creek coal field.f 



The rim of the syncline is in large part formed by the great 

 Carboniferous limestone which also apparently underlies much 

 of the central part of the basin, but it is effectually concealed 

 by a great mass of later rocks consisting of lava, tuff, and beds 

 of sandstone and shale. These beds are probably of late Cre- 

 taceous age, and, therefore, there is a great time-break between 

 them and the underlying limestone, although in most cases the 

 beds at the contact are apparently conformable. 



The rocks immediately overlying the limestone generally 

 consist of sandstone and shale with some small coal beds. 

 Above this group there are many beds of andesitic tuff with 

 iuterbedded sand and clay and what appears to have been 

 great surface-flows of andesite. Some 500 or 600 feet above 

 the base of the Cretaceous rocks is a bed of conglomerate 

 composed of bowlders of all sizes up to 2 or 3 feet in diameter, 

 held in a matrix of very soft andesitic tuff, a typical outcrop 

 of which is shown in figure 1. This view was taken on Ash 

 Creek just below the limestone box canyon east of Saddle 

 Mountain and the bedding planes of the conglomerate dip 70 

 degrees to the left, or toward the center of the basin. The 

 bowlders represent a great variety of rocks, both crystalline 

 and sedimentary, but probably those of quartzite are most 

 abundant. 



This bed of conglomerate shows at a great many points, but 

 in most places the matrix is so soft that the rock breaks down 



* Published by permission of the Director of the U. S. Geological Survey. 



fFor a more detailed description of the region and the geologic relations 

 see ''The Deer Creek Coal Field, Arizona," by the writer. U. S. Geol. 

 Surv., Bull. 225, pp. 240-258. 



