126 REPORT UNITED STATES GEOLOGICAL SURVEY. 



out being able properly to account for their presence. It was found that 

 several of the dumps were ignited, a common condition due to spontaneous 

 combustion of the coal, which is produced by the decomposition of pyrite. 

 In the same manner probably the coal in situ took lire and burned so long 

 as the supply of oxygen could sustain a flame. Through this process of 

 metamorphosis by heat the overlying beds containing more or less 

 hydrated ferric oxide were changed to a bright vermilion color. We 

 found sandstones, the faces and edges of which had been literally glazed 

 by the long-continued action of heat. Fragments were firmly baked 

 together, and resembled cinders from a furnace. Purely argillaceous 

 shales and clays had been thoroughly fritted, and were altered into very 

 hard, compact porcelain jasper. When found in small pieces, this latter 

 is generally red on the outside surface, but upon being broken it is of a 

 greenish-blue color. This difference is due to coloring produced in the 

 first instance by ferric and the second by ferrous oxides. Prof. J. A. 

 Allen, of Cambridge, has observed similar occurrences, and has published 

 a highly-interesting paper upon the subject.* Throughout the area cov- 

 ered by the Laramie Group, and in some of the Wasatch beds, we found 

 the same red color of strata produced by the same causes. 



In the vicinity of Salt Wells we observed the Laramie Group again 

 exposed. It there follows a line of outcrop similar to that shown by the 

 Pox Hills. Dipping in three directions, the exposure to the north occu- 

 pies the smallest area. On the west side the beds dip off steeply from 

 the Pox Hills, following a strike which carries them to Pock Springs. 

 Here large masses of oysters were found along the slopes of small 

 bluffs. They were determined as Ostrea Wyomingensis and 0. glabra. 

 Coal-beds are found here in the Laramie Group, a short distance above 

 the Cretaceous strata. Their outcrops were too much decomposed to 

 afford any material for analysis. I here insert an assay made by Pro- 

 fessor Frazer, and published in the report for 1870. The coal is from 

 Bock Springs, and there occurs as a direct continuation of the beds we 

 observed northwest of Salt Wells : 



Per cent. 



Volatile matter 42.62 



Coke 54.88 



Ash 11.00 



Sulphur 00.50 



100. 00 



Following the Laramie beds along their strike, we are carried to the 

 eastward and then southeast in a line reaching Point of Eocks. An 

 analysis made by Professor Frazer of. coal from this place yielded him : 



Per cent. 



Moisture 8.54 



Volatile matter 30.60 



Carbon 52.34 



Ash, white 8.52 



100. 00 



From a comparison of these results with those obtained by W. Ash- 

 burner, who analyzed some coal from Separation, it will be found that 



* Metamorphism produced by the burning of lignite beds in Dakota and Montana. 

 J. A. Allen, Boston, 1874. 



