8G GEOLOGY OF THE EUREKA DISTRICT. 



broad masses they resemble the Upper Nevada hmestoiie, but are rather 

 lig-hter in color in distinction from the dark blue and black of the latter 

 horizon. No true dolomite beds of any con.siderable thickness have been 

 recognized, 9 '21 per cent being- the largest amount of magnesium cai'bon- 

 ate obtained in any of the rocks subjected to chemical analysis. Across 

 their broadest development they measure about 3,800 feet in thickness, 

 which is much less than has usually been assigned to this horizon in other 

 mountain uplifts, more especially those lying eastward. 



As the term Lower Coal-measure has been emploj'ed by most geolo- 

 gists to designate this epoch throughout the Great Basin, it has been 

 thought best to retain the name provisionally, although not exactly appli- 

 cable, as the epoch includes such a commingling of species from both the 

 Upper and Lower Coal-measures that a separation of the beds seems quite 

 impossible. Moreover, those distinctions which hold good in the Missis- 

 sippi Valley are by no means always applicable to the Cordillera. In the 

 present state of our knowledge of the Carboniferous limestone, it is impos- 

 sible to establish subdivisions in either of the Coal-measure epochs, based 

 upon faunal differences, owing to the fact that so many species extend 

 thi'ough a wide vertical range, and so few characteristic species occur within 

 restricted limits. 



Lower Coal-measure Fauna.— As the limestones are in general favorable to 

 the preservation of organic remains, fossil-bearing strata are found through- 

 out the beds, and geologists are not so dejjendent u[)ou definite horizons 

 as among Lower Paleozoic rocks. About 100 species have been collected 

 from this epoch, but most of those obtained from the upper and middle 

 portions have already been recognized as occurring elsewhere in the 

 Lower Coal-measures of the Great Basin. In comparison with the new 

 species obtained from the Cambrian, Silurian and Devonian, the Carbon- 

 iferous of Eureka offer singularly few forms new to science, but this, of 

 course, may be accounted for by the thorough researches which have been 

 made in this period elsewhere. At the base of the limestone the life is 

 more varied and presents certain facts that are of both geological and 

 biological interest. 



Three salient features in the life of the Lower Coal-measures at Eureka 



