2 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 8l 



specifs herein described remain in the National ^Museum, and a third 

 set was set to Princeton University. These fossils are interesting in 

 that they constitute the most southerly and westerly occurrence of 

 Cambrian beds west of the Rocky Mountains, besides being sufficiently 

 well preserved to show structural features, particularly of the 

 Mesonacidae which are yet but poorly and incorrectly described. 



GEOLOGICAL SECTION AND CORRELATIONS 



The section in Bristol ^Mountain, as described by Clark, may be 

 summarized as follows: about 470 feet (143.3 "''■) o^ cjuartzites rest 

 unconformably on an eroded granite surface. Above this lied occurs 

 about 22 feet (6.7 m.) of fine-grained, arenaceous shale containing 

 thin beds of sandstone. The Mesonacid fauna herein described occurs 

 abundantly in the soft shale portion of this bed. Xext above is 25 feet 

 (7.6 m.) of blue to black, unfossiliferous, nodular limestone. This is 

 in turn overlain by 120 feet (36.6 m.) of brown or black arenaceous 

 shale from which, about 12 feet {'i^.y m.) below the top. the two ^liddle 

 Cambrian fossils were obtained. The Paleozoic section is terminated 

 by Carboniferous limestones about 635 feet (193.5 ^''''•) thick. 



Owing" to the presence of the Mesonacidae, the three lower beds 

 were referred to the Lower Cambrian, and the overlying shale to the 

 Middle Cambrian. No question can be raised as to the Middle Cam- 

 brian age of the fossils in the upper shale, but the final decision as 

 to the Mesonacid fauna must await the results of studies now being 

 made at many places in an attempt to settle the vexing question as to 

 where the Lower-Middle Cambrian boundary must be drawn. 



According to our present ideas, Bristol Mountain must be in the 

 seaway through which various Cambrian seas are supposed to have 

 invaded the continent from the Pacific. Both faunas here described 

 would appear to be more or less closely related to those in formations 

 elsewhere in the southwestern United States, all of which were 

 deposited in shallow seas whose exact extent and connections are 

 not yet fully known. The fossils in the lower shale find their nearest 

 affinities in the Prospect Mountain formation far to the northeast 

 in the Eureka District, Nevada, with some relationship also apparent 

 in the intervening Silver Peak District. The seas in which these older 

 beds in the three regions mentioned were deposited certainly had 

 Arctic connections, whereas no faunas are at present known from 

 beds deposited in strictly Pacific seas. Whether the occurrence of 

 older Cambrian beds in Bristol Mountain indicates Pacific connections 

 must remain undetermined for the present. However, it would appear 



