review of data determixixg time range. 459 



Time Range of the Series. 



The recent investigations of H^^att and Smith alreach^ referred to 

 assign the Mariposa beds to late, though not the latest, Jurassic, while 

 the paleontologic evidence we have given indicates that the Knoxville 

 includes the earliest Cretaceous deposits. It is evident from the faunal 

 relations of these two formations that their epochs of deposition were not 

 sej^arated by a long time interval, but it appears that during this interval 

 the upturning of the Mariposa beds occurred, and some time must have 

 elapsed. 



The deposition of that series continued Avithout a general interruption 

 from the beginning of the Knoxville to the close of the Chico, and the 

 time represented by the 30,000 feet of strata which accumulated during 

 that interval must necessarily be long. 



The Ammonites of the upper part of the Horsetown to which reference 

 was made in the note on the fauna are either identical with or closely 

 related to forms which in Europe are characteristic of the uj)pe.r Gault 

 and lowest Cenomanian. Ammonites inflrdus, especially, is there regarded 

 as the characteristic fossil of a zone that is now placed at the base of the 

 Cenomanian. The other invertebrate fossils from these beds do not con- 

 flict with this evidence '^ of the Ammonites, and it is therefore safe to say 

 that the Knoxville and Horsetown together represent all of the Lower 

 Cretaceous — that is, everything from the Wealden and Neocomian to the 

 Gault, inclusive, with perhaps a small portion of the Upper Cretaceous. 



It is somewhat more difficult to fix the time range of the Chico. Its 

 basal portions must have been deposited early in Upper Cretaceous time, 

 but the time of its close is more uncertain. Its large and varied littoral 

 fauna, with a few Ammonites, seems to have no species in common with 

 the American Cretaceous faunas east of the Sierra Nevada, though it is 

 true that several genera are represented in the Montana and Ripley 

 formations (of Senonian age) by similar species ; but the known American 

 faunas corresponding to the European Cenomanian and Turonian are 

 comparatively small and do not offer mucli l)asis for comparison. As an 

 exception to this general statement we may cite a recently discovered 

 littoral facies of the Fort Benton fauna, locally developed in southern 



♦Professor Fortaine's stadj' of the small collection of plants taken from several liorizons ranging 

 from within the Knoxville to near the top of the Horsetown shows that they are ail related to forms 

 in the floras of the Wealden, the Trinity and the Potomac, all of which he regards as oldest Cre- 

 taceous. The testimony of the invertebrates agrees with this in a general way, excepting that 

 they show a later age for the upper part of the series. In such comparisons it is well to remember 

 that the known plant-bearing beds do not form as complete and continuous a standard series as is 

 furnished by those containing marine invertebrates, and for this reason correlations made on the 

 evidence of plants alone are frequently not as definite as may be made by the Ammonites, for 

 example. 



j,IV_BL-i,r,. Geol. Soc Am., Voi,. 5, 1893. 



