DIKES PREVIOUSLY DESCRIBED 265 



According to Doctor Pavlow, this dike is an isolated fragment of Ter- 

 tiary sediments caught in a crevice formed in the Cretaceous clays when 

 the Tertiary sea covered the region. The Tertiary sea retreated later, 

 and all the other sediments of Tertiary age were removed by erosion 

 prior to the deposition of the alluvial material that now covers the 

 region, leaving the dike as the only evidence that the Tertiary sea once 

 covered the locality in which it occurs. 



Ashley, 1897. In the proceedings of the Indiana Academy of Science 

 for 1897, pages 242-250, Dr George H. Ashley describes and figures a num- 

 ber of " clay veins " observed in the coal fields of Indiana. He also re- 

 fers to many " clay veins " in his report on the coal fields of Indiana in 

 the twenty- third annual report of the Indiana Geological Survey. 



Crosby, 1897. In Science for April 16, 1897,^ Professor W. 0. Crosby 

 gives an abstract of a paper previously read before the Boston Society of 

 Natural History on the Great fault and accompanying sandstone dikes 

 of Ute pass, Colorado, in which he calls attenion to the salient features 

 of the dikes previously described by Cross, and also to the continuation 

 of those dikes to the southeast. Regarding the distribution of the dikes 

 studied by him, Professor Crosby says : 



" I have succeeded in tracing the sandstone dikes from the vicinity of the Iron 

 spring, in Manitou, northwest along the Great fault 2 miles, or a little farther than 

 the sedimentary rocks extend, and southeastward from Manitou along the base of 

 the mountains, and closely following the Ute fault, to Cheyenne canyon and be- 

 yond, a distance of 6 miles. The dikes of this series vary in width up to 500 or 

 600 feet. A large dike usually follows the main line of displacement, separating 

 the sedimentary rocks and granite, with one to several other dikes closely parallel 

 with it in the granite." 



The dikes have usually a southwesterly hade of from 5 degrees to 75 

 degrees from the vertical, and often present '' slickensided shear planes 

 at corresponding angles." 



Regarding the material of these dikes, which vary in thickness from 

 mere films to the great masses noted above, he says : 



"Although the rock is prevailingly a fine and even grained gray and reddish 

 brown sandstone, identical with that described by Cross, much of it is decidedly 

 coarser, and at several points it is distinctly conglomerate. In several dikes, also, 

 the sandstone is more or less distinctly stratified." 



Crosby is of the opinion that the dikes date from the formation of the 

 Ute fault, with which they are so closely associated ; that fault fissures 

 were filled by overlying, unconsolidated sands passing down into the 

 fissures at the time of faulting, and that the extremely thick masses of 

 sand represent areas where " sheets of granite of varying width and bor- 



* Science, new series, vol. v, 1897, pp. 604-607. 



