WEAVER: INVERTEBRATE PALEONTOLOGY AND HISTORICAL GEOLOGY 729 



glaciers in the Alps, considered that at an earlier time these glaciers had ex- 

 tended out on to the plains, where they formed great ice sheets, and that such 

 conditions had occurred over large areas of the continent. The Diluvium of 

 northern Europe is largely of glacial origin and the Quaternary was thought to 

 have opened as continental glaciation began. In later years Quaternary time 

 was divided into Pleistocene and Recent, the latter representing the interval 

 between the last withdrawal of the ice and the present. Usually this interval 

 has been considered to be about 25,000 years, but carbon^* studies indicate that 

 it may be only slightly more than 10,000 years. In certain parts of the world, as 

 in Greenland and Antarctica, continental ice still persists, whereas in the tropi- 

 cal areas, except at high altitudes, it never existed even during the Pleistocene. 



Long continued investigations in North America by Alden, Antevs, Cham- 

 berlain, De Geer, Leverett, Bretz, Matthes, Flint, and others have led to the 

 recognition of four glacial epochs, during which the ice sheets advanced south- 

 ward from Labrador and north central Canada halfway down into the United 

 States, and three interglacial epochs, when the ice completely retreated leaving 

 the surface which it had occupied covered with debris carried in and on the ice 

 from northern regions. Each successive glacial deposit rests unconformably 

 upon the much-weathered and eroded surface of the one beneath and it was 

 largely from the interpretation of such data that distinct glacial intervals were 

 recognized. Such studies have made possible an interpretation of the geologic 

 history of the Quaternary. 



Regions in lower latitudes which were only indirectly affected by glaciation 

 have a history characterized by erosion, deformation, and accumulation of sedi- 

 ments similar to that of the Pliocene and older epochs, as in the Coast Ranges 

 of California where thick deposits of Pleistocene and Recent sands, clays, and 

 gravels of both marine and continental origin accumulated. In many places 

 these strata have been folded and faulted, with evidence of local diastrophism 

 during the Pleistocene. 



Submarine Investigations : During the past twenty-five years intensive geo- 

 logical studies of the floor of the ocean have been initiated by F. P. Shepard, 

 Maurice Ewing, and Ph. II. Kuenen, and their students. This work has demon- 

 strated that many of the classic concepts of the ocean floor and of geologic 

 processes in the ocean are based on inadequate data and need to be revised. 

 Scholars are now making highly significant discoveries and have demonstrated 

 the existence of great submarine valleys, mountain chains, and escarpments, 

 and have found fossiliferous materials of Cretaceous and Tertiary ages in areas 

 far distant from present land areas. Many important developments affecting 

 historical geology may be expected from this field in the near future. 



SELECTED BIBLIOGRAPHY 

 AoAssiz, Louis 



1833-1844. Recherches sur les poissons fossiles. 6 vols. Neuchatel. 



1840. fitudes sur les graciers. 346 pp., 32 pis. Neuchatel. 

 Aluerti, F. a. von 



1834. Beitrag zu einer Monographie des Bunten Sandsteins, Muschelkalks und 

 Keupers, und die Verbindung dieser Gebilde zu einer Formation, xx + 366 pp., 

 2 pis. Stuttgart and Tubingen. 



