Geology and Mineralogy. 143 



thus demonstrating the presence of the Upper Lias at these 

 three localities, two of them separated by the crests of the 

 Sierra Nevada. 



Tipper Jura. 



The Gold Belt fossils were next taken up and discussed. The 

 discovery of Gabb's type of Ammonites Colfaxi showed that it 

 was a true Perisphinctes and with other forms of the same genus 

 recently found near Colfax demonstrated the Upper Jurassic age 

 of the rocks on the western slope of the Sierras. The Aucellae, 

 and the Ammonitinae of the genera. Cardioceras and Perisphinctes, 

 from Mariposa, Calaveras and Tuolumne Counties were repre- 

 sented in collections acquired in the last few years by the U. S. 

 Geological Survey and proved, in his opinion, the Upper Juras- 

 sic age of the gold belt series of slates. The species of Cardio- 

 ceras and Perisphinctes had the common characteristics of the 

 similar forms found in the Upper Jura of the Russian fauna and 

 these occurred in association with similar forms of Aucellae. 



The species of Aucellae differed as a whole from those of the 

 Knoxville slates in being almost invariably ornamented with 

 radiating striae. Only one species, the A. Errington var. arcuata, 

 approximated in outline to the well-known, narrow form of 

 Aucella Piochi. This is invariably smooth and as lately demon- 

 strated by Diller and Stanton occurs only in the lower part of 

 the Knoxville slates. The conclusion was thus reached that the 

 fauna of the gold belt series of rocks was Upper Jurassic and 

 younger than that of the Knoxville series. (Author's abstract). 



Henry S. Williams read a paper on Dual nomenclature in 

 Geological classifications, setting forth the importance of differ- 

 entiating between the lists of geological formations and the divi- 

 sions of the time scale. He showed that the former are defined 

 and determined by physical, structural and mineral conditions 

 which are always local and never uniform for any considerable 

 geographical extent ; and that the latter, however indefinitely, 

 are always known and defined by particular species or genera of 

 organisms which had a definite life period and hence record an 

 actual determinable point in time. 



The imperfection of the present method was illustrated by the 

 case of the Catskill formation, whose place in the scale, and the 

 use of the name has recently been discussed by Hall, Darton, 

 Stevenson. Prosser and the author. The author emphasized the 

 fact that in our common usage there are two entirely distinct 

 sets of facts confused in the nomenclature and classification, called 

 the geological scale; that the confusion and evils arising there- 

 from are not corrected by merely providing two sets of nomen- 

 clature, as the international congress of geologists has done in 

 proposing group, system, series, stage with their corresponding 

 chronological terms, era, period, epoch, age, so long as there is 

 only a single set of divisions classified. The fact is that the 

 stratigraphical division planes used in defining the formation- 



