264 TEN years' progress in vertebrate paleontology 



America or change our ideas regarding Cretaceous mammalian types. 

 The Santa Cruz formation has been referred to the Eocene, Oligocene, 

 and Miocene, and the Patagonian, Pyrotherium, and other less well 

 established intervening beds shifted about at will from Cretaceous to 

 Tertiary. Fortunately, Dr. F. W. True has found a clew to the mystery 

 in the identification of the same genera of whales in the Patagonian for- 

 mation of Argentina as in the Chesapeake Miocene of our Atlantic coast. 

 Omitting South ilmerican horizons, the chronologic and correlation 

 standards of the "Age of Mammals" are now in use by all American and 

 European workers in vertebrate paleontolog}'. 



The most important recent contribution to geologic theory which may 

 be credited in part to vertebrate paleontology has been the substitution 

 of seolian and fluviatile agents for lacustrine in explaining the origin of 

 continental Tertiary formations. Matthew^s paper, "Is the White Eiver 

 Tertiary an ^^olian Formation?" (1899), was, in this respect, epoch- 

 making. It was followed by contributions from other workers, both 

 geologists and paleontologists, so that now, of the numerous fresh-water 

 seas which used to spread so extensively over our geologic maps and 

 ^through the pages of our text-books, only the Florissant and Green Elver 

 lakes and the white layers in the Bridger remain. Evidence from many 

 sources, dealing with the nature of the fossils and the conditions con- 

 trolling their intombment and preservation, changes in faunal facies 

 with changing lithology, the stratigraphic interrelations of different por- 

 tions of the fossil-bearing sediment, the origin of the sedimentary mate- 

 rial, etcetera, was presented, but it was not until the problem was at- 

 tacked by the methods of microscopic petrography that the lacustrine 

 theory was finally abandoned. Calkins's "Contributions to the Petrogra- 

 phy of the John Day Basin," published in 1902, is the first paper dealing 

 with the microscopic petrography of formations affording fossil verte- 

 brates, and although a great field was thereby opened for the investiga- 

 tion of epicontinental sedimentation in its relation to diastrophism, 

 structure, and life conditions, few have been sufficiently interested to 

 continue these investigations. Of the three papers which have since ap- 

 peared dealing with the lithology of western Tertiary formations, none 

 has been by a petrologic expert. The important part played by volcanic 

 ash in these accumulations has only recently been realized. We now 

 know that the Santa Cruz formation of Patagonia is composed entirely, 

 or ajmost entirely, of ash, while of N"orth American horizons, the 

 Bridger, Washakie, John Day, Mascall, Eattlesnake, a large part of the 

 White Eiver, and probably others not yet investigated, are either in large 

 part or entirely ash. Ash layers have also recently been reported in the 



