88 GEOLOGY OP THE EUREKA DISTRICT. 



class which heretofore has been but sparingly represented ill the collections 

 of Carboniferous fossils from Utah and Nevada. Indeed, all told, there 

 have been but few species recognized from the Paleozoic of the Great 

 Basin. Most of those collected at Eureka are new species, described for 

 the first time, but allied to forms found in the Mississippi Valley and At- 

 lantic States, while others appear to be identical with well known species 

 A complete catalogue of the Lamellibranchiates will be found under the 

 lists of Devonian and Carboniferous species in an appendix at the end of 

 this volume. 



Commingling of Carboniferous Species. Prof. R. P. Wllitfield aild Dr. C. A. 



White have frequently called attention to the commingling of Lower Car- 

 boniferous and Coal-measure species in New Mexico, Colorado and Utah 

 which, in the Mississippi Valley, are quite distinct and regarded as char- 

 acteristic of one or the other of the two horizons. So far as known to 

 the writer nowhere is this commingling of types more strikingly brought 

 out than at Eureka. Moreover, here they are associated with species 

 which, in New York and Ohio, are regarded as typical of the Devonian, 

 several of them being restricted within a very limited vertical range. 

 This grouping of fossils is found on a low hill on the west base of 

 Spring Hill, a long monotonous ridge lying just to the east of 

 the Hoosac fault and made up wholly of Lower Coal-measure strata. 

 The beds of Spring Hill Ridge, along the fault, for the most part dip 

 toward the east. On a small but prominent outlying hill on the western 

 slope of the ridge they lie inclined toward the west, the result of an anti- 

 clinal fold within the main body of limestone. In this outlying hill occurs 

 a well marked bed of arenaceous limestone dipping about 50 to the west 

 towards the Hoosac fault and cropping out both on the east and west slopes 

 of the hill; the same bed being recognized in the main ridge on the 

 opposite side of the anticline. This limestone, which has been traced for 

 short distances, both north and south, has furnished a most varied fauna. 

 Owing to its paleoutological importance, Mr. Walcott has given especial 

 attention to the group and has distinguished over fifty forms, most of which 

 he has specifically determined. About one-third of them he regards as ideu- 



