PALEOZOIC STRATIGRAPHY OF ROCKY MOUNTAINS 255 



Members 2-4. — There can be little doubt as to the validity of 

 most of the correlations of Trenton strata shown in the preceding 

 table and diagrams, except in Utah and Nevada. The main mas- 

 sive member (No. 4) of the Lower Bighorn, with its accompanying 

 basal weaker strata (Members 2 and 3) and its cap of smooth, 

 nearly chalky, dolomite, constitutes a highly characteristic and 

 almost unique series. Furthermore, Member 2 is fossiliferous, 

 and Member 4 is sparingly so in nearly all localities. 



The upper half of the massive member in the Blacksmith Fork 

 section, which the writer has called the Lower Fish Haven dolomite, 

 is similar in all essential respects to the massive Trenton member 

 of the typical Bighorn. It is marked off from the Richmond above I 

 by a conglomerate, and disconformity at its base is sufficiently 

 indicated by the absence of the (Chazyan ?) quartzite which inter- | 

 venes at that horizon elsewhere in northern Utah. This lower 

 Fish Haven dolomite carries Halysites, which is not known from 

 rocks older than Mohawkian. 



The lower part of the Lone Mountain limestone, which uncon- 

 formably overlies the Eureka quartzite in western Nevada, carries 

 a fauna assigned by Walcott 1 to the Trenton. Ulrich 2 has voiced 

 the opinion that part of the Lone Mountain limestone is older than 

 the Bighorn dolomite; but it is probable that the former formation 

 contains a representative of the Trenton series. 



Members 5-7; the Leigh formation. — Members 5-7 of the Middle 

 and Upper Ordovician series constitute a distinct and very widely 

 developed unit. In the Goose Creek Ridge section there is little 

 ground for differentiating these three from each other; but the 

 lowest Richmond fauna occurs in the beds there marked as Member 

 6. In the Crandall Creek and Dead Indian Creek sections there is 

 a conspicuous surface of disconformity, with a basal breccia, at 

 the base of Member 6. As this is the only disconformity for which 

 physical evidence has so far been noted anywhere within the limits 

 of the Bighorn formation, and as it coincides (by lithologic correla- 



1 Arnold Hague, op. cit., pp. 61-62, 196-97; also appendix by C. D. Walcott, 

 pp. 324-25- 



2 Cf. Bailey Willis, "Index to the Stratigraphy of North America," U.S. Geol. 

 Survey, Prof. Paper 71 (1912), p. 169. 



