Page:Popular Science Monthly Volume 37.djvu/115

Rh land-shells, and its generally unstratified character, owes its origin to rain, frost, and wind. Admitting that some of the loess of the lower grounds may have been reworked by the same agents, Prof. Geikie found no evidence in the facts adduced by German geologists of a "dry-as-dust" epoch having obtained in Europe during any stage of the Pleistocene period.

Within recent years the fossils of the loess have received close attention, and through them so much knowledge has been gained of the various modifications experienced by Pleistocene organisms that, taken with other evidence of interglacial conditions, there is little room to doubt that this period was characterized by great changes of climate. How often arctic, steppe, prairie, and forest faunas and floras have replaced each other is yet a matter of dispute. The occurrence of fossiliferous deposits intercalated among glacial accumulations throughout all the glaciated tracts of Europe show that however many advances and retreats of the ice there may have been, they were on a gigantic scale characterizing all the glaciated areas.

The bearing of the establishment of at least two eras of glaciation on the position of Palæolithic man was pointed out by Prof. Geikie. The mere occurrence of glacial deposits underneath implement-bearing beds no longer proves these latter to be post-glacial. The horizon of glacial accumulations underlying Palæolithic gravels must now be determined by ascertaining their relative position; and it is a remarkable fact that the bowlder-clays which occur beneath such old alluvia belong, without exception, to the earlier stages of the Glacial period. In 1871–'72 Prof. Geikie published a series of papers in the Geological Magazine, maintaining that the alluvial and cave deposits must be assigned to preglacial and. interglacial times, and in chief to the latter. Evidence was adduced to show that during the last stage of the Glacial period man lived contemporaneously with a northern and Alpine fauna, in such regions as southern France; and that Palæolithic man and the southern mammalia never revisited northwestern Europe after extreme glacial conditions had disappeared. Prof. Geikie at the same time colored a map to show at once the areas covered by the glacial and fluvio-glacial deposits of the last Glacial era, and the districts in which the implement-bearing and ossiferous alluvia had been found; and this clearly brought out that the latter never occurred at the surface within the regions occupied by the former. Similar evidence has been recently obtained by continental geologists; and a map published by Dr. Penck in 1884, showing the areas covered by the earlier and later glacial deposits in northern Europe and the Alpine lands, and indicating at the same time the various localities where Palæolithic finds have occurred, does not give a single