598 THE JOURNAL OF GEOLOGY. 



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 Car- 

 boniferous 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 sup- 

 posed colony mentioned by C. R. Keyes 1 from the Burlington of 

 Missouri, and observed in Arkansas by the Geological Survey of 

 Arkansas. 2 In both places, in the midst of 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 fos- 

 sil in Europe, but in America is not found east of this place, 

 unless P. latissimus, Sowerby, which F. B. Meek 3 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 4 as evi- 



'Am, Jour. Science, December, 1892, p. 447. 

 2 Journal of Geology, Vol. II., p. 198. 



3 Bull. U. S. Geol. Survey Terr., Vol. II., No. 4, p. 354. 



4 Journal of Geology, Vol. II., p. 200. 



