ROTO RAIN GEA ECLALE DP POSITS. 243 
the other by a prolonged epoch of erosion and accumulation, 
under climatic conditions somewhat similar to those now existing. 
Meanwhile, geologists in Britain had discovered that the drift 
deposits formed a triple seriles—namely, (a) tower bowlder- 
clay, (6) middle sand and gravel, and (c) upper bowlder- 
clay. And some were inclined to consider this grouping as 
roughly corresponding to the threefold arrangement of the 
Swiss deposits, and to infer that in the British area there had 
been two glacial epochs separated by an interval of submergence, 
when somewhat milder climatic conditions prevailed. In Ger- 
many, likewise, an ‘“‘upper’’ and a “lower” Diluvium had been 
recognized long before the true significance of these groups 
dawned upon geologists. As observations increased it was found 
that here and there, in Scotland and elsewhere, beds of peat and 
fossiliferous fresh-water deposits appeared intercalated in bowlder- 
clay, or separating a lower from an upper bowlder-clay. By 
those who were of opinion that bowlder-clay is a ground moraine 
these intercalated beds were looked upon as evidence of glacial 
oscillations, of no great magnitude. It was supposed that the 
former glaciers and ice-sheets were subject, like their modern 
representatives, to temporary movements of advance and retreat. 
During a time of retreat vegetation spread over the ground 
vacated by the ice, and when the next forward movement took 
place the glaciers reoccupied the area invaded by plants, and 
buried the old soils under fresh accumulations of glacial débris. 
The upholders of the iceberg origin of the drifts did not attempt 
to account for the appearance of such intercalated fresh-water 
beds. It was easier to belittle their importance or to ignore 
them altogether. The deposits in question have now been 
encountered, however, in so many formerly glaciated areas that 
it is no longer possible to pass them by as accidental occurrences 
that may in time be somehow or other explained away. Their 
true significance is now the only question in dispute. Do they 
indicate mere local and temporary movements of glacial retreat 
and advance, or are they relics of long-continued milder con- 
ditions occurring between separate and distinct glacial epochs? 
