CAEBONIFEROUS UNDIVIDED. 375 



conformably by the McCloud limestone, but the contact could not be observed, and the diabase 

 which separates the two divisions may mark an unconformity. 



The list of fossils from the Baird contains a total of 84 species, of which 19 were not specifi- 

 cally identified. Out of the 65 forms specifically determined 26 occur in the Waverly of the 

 Mississippi VaUey, although 8 of these also occur in the Upper Carboniferous of that region. 

 Fifteen are known to occur in the Devonian of the Eastern States, but of these 6 also occur 

 in the Waverly; thus there are only 9 forms that in the Mississippi Valley, or east of it, 

 would be considered as decidedly Devonian. Thirty-six are known from the Waverly and 

 Lower Carboniferous of Utah, Nevada, and New Mexico; of these 29 correspond to forms 

 described by Walcott from the Lower Carboniferous of the Eureka district, Nevada. Three 

 species, Aviculopecten cahoniferus, Aviculopecten interlineatus, and Macrodon tenuistriatus, have 

 been considered characteristic of Upper Carboniferous. 



With this assemblage of species one would not hesitate to place these strata low down in 

 the Carboniferous, but whether they are equivalent to the Waverly is a question not so easily 

 settled. Although about one-half of the species are found in the Waverly and nearly one- 

 fourth in the Devonian of the Mississippi Valley, 29 of these and a large number of others as 

 yet unknown in California, were found in the Eureka district, Nevada, in strata of Lower 

 Carboniferous age but lying 3,000 feet above the Upper Devonian White Pine shale. 



We do not know the age of the rocks immediately underlying the Baird shales, but the 

 siliceous shales of the Sacramento River lie some distance below them and are probably in part 

 of Carboniferous age. It thus becomes probable that in California, as in Nevada, the Waverly 

 fauna, with a few Devonian forms, lived on after the corresponding faunas had become extinct 

 in the eastern region. A migration of these survivors into the Lower Carboniferous sea of the 

 Mississippi Valley may explain the supposed colony mentioned by C. R. Keyes from the Bur- 

 lington of Missouri, and observed in Arkansas by the Geological Survey of Arkansas. In 

 both places, in the midst of the undoubted Lower Carboniferous faunas, there appears a group 

 of fossils that if found alone would be classed as of Waverly age. They are not colonies in the 

 sense in which Barrande used that word but are simply migrations from one faunal region into 

 another, due to shifting of physical barriers; these migrations have taken place during all time 

 and have complicated correlations, until we lose faith not only in the idea of synchronism as 

 proved by fossils, but also in homotaxis, unless we can find the direction of the migration. 



In the paleontological sense the Baird shales are homotaxial with the Waverly, while 

 stratigraphically they probably are not but would agree more nearly in position with the higher 

 divisions of the Lower Carboniferous of the Mississippi Valley. 



The occurrence of Productus giganteus Martin in these strata is very interesting. This is 

 a common Lower Carboniferous fossil in Europe, but in America is not found east of this place, 

 unless P. latissimus Sowerby, which F. B. Meek has cited from Montana, on the western slope 

 of the Rocky Mountains, is an equivalent of it. This fact has been used by the writer as evidence 

 that the European Carboniferous species found in America migrated through the ocean that 

 connected on the west the American with the European Carboniferous waters. 



Immediately above the Baird shales, and probably conformably with them, lies the McCloud 

 hmestone. This series is about 2,000 feet in thickness, uniform in bedding, and very siliceous 

 in places. Some few beds are altered to a crystalline marble, but in the mam the series is made 

 up of a fine-grained hard gray limestone, which at the base contains few fossils besides corals, 

 ClisiofhyUum gabhi Meek, and Lithostrotion californiense Meek. But toward the top the beds 

 become more fossiliferous and contain a varied assemblage of species, which, however, do not 

 rival in number those of the Baird shales. 



Taken by itself the fauna [of the "McCloud formation"] would not be characteristic of 

 Upper Carboniferous, and indeed it is arbitrary to draw the line at the base of the limestone. 

 Even Fusulina cylindrica, which in the region east of the Rocky Mountains seems to be char- 

 acteristic of Upper Carboniferous, in Nevada is found also in Lower Carboniferous. But the 



