DIKES PREVIOUSLY DESCRIBED 263 



The deposits described occur as veins or dikes in clays. According to 

 Hilgard, the deposits are composed of bitumen, mixed "with 75 to 80 

 per cent of a fine siliceous clay, substantially identical with the wall 

 material and main mass of the formation in which it occurs." The dikes 

 vary greatly in thickness and give off branches and at some places con- 

 tain large pockets, and Hilgard believes them to be intrusions from below, 

 having been forced by pressure into fractures in the overlying clays in 

 which they are now found. 



McGee, 1900. On page 440, volume 1, Bulletin of the Geological Society 

 of America, Mr Diller mentions the discovery by W J McGee of a 

 number of small sandstone dikes which intersect Eocene strata at 

 Corinne, Mississippi. It is also noted that some years previous to 1890 

 Mr C. D. Walcott had collected a fi'agment of a sandstone dike inter- 

 secting limestone at lake Champlain. 



Hay, 1891. In 1891 Robert Hay described two sandstone dikes in 

 northwestern Nebraska, 2 miles southward from the town of Cadrow.-'^ 

 The dikes are half a mile apart and are regarded by Hay as having been 

 " intruded before the completion of the deposit of the soft clays and 

 marls " of the White River beds in which they occur. The intrusions 

 stand up in wall-like outcrops, being harder than the inclosing rocks. 

 Regarding their origin Hay says : 



" These dikes may be related to the phenomena of mud volcanoes as they were 

 certainly intruded from below." 



Keyes, 1893. C. R. Keyes mentions the occurrence of clays and sand- 

 stones filling fissures in the Coal Measures of Iowa in 1893. These dikes 

 are usualh^ from a few inches to a foot or more thick, and " more or less 

 vertical, with very irregular borders." One such intrusion is figured. f 



Stone, 1893. In 1893 Professor George H, Stone pointed out the fact 

 that the " Pinkeye lode" in the Turkey creek mining district, south- 

 west of Pikes peak, is a dike of sandstone intersecting granite gneiss ; 

 the dike trends almost north and south. J 



Abbott, 1894. Mr W. .J. Lewis Abbott described fissures filled with fos- 

 siliferous fi'agmental materials, near Ightham, Kent, in 1894,§ Mr Abbott 

 believes that " the deposit was introduced into the fissures by a river." 



Cross, 1894. In 1894 Whitman Cross described ' a number of sand- 



* Robert Hay: Sandstone dikes in northwestern >'ebraska. Bull. Geol. Soc. Am., vol. 3, 1892 

 pp. 50-55. 



t Charles Rollin Keyes: Crustal adjustment in the upper Mississippi. Bull. Geol. Soc. Am., 

 vol. 5, 1893, p. 2;i9. 



t George H. Stone : The Turkey Creek mining district, El Paso county, Colorado. Eng. and 

 Min. Jour., vol. Ivi, 1893, p. 262. 



g Quart. Jour. Geol. Soc. of London, vol. 40, 1S94, pp. 171-lST. A list of vertebrate fossils from 

 hese fissures is given in an accompanying paper by Mr E. T. Newton. 



Whitman Cross: Intrusive sandstone dikes in granite. Bull. Geol. Soc. Am., vol.5, 1893 

 pp. 225-2.30. 



