1878.] 
Glacial Drifts in North America . 
67 
the northern ice. We have thus two distindl phases of the 
glacial era clearly marked in North-eastern America, — a 
Glacier period and an Iceberg period, just as we have in 
Europe. They are distinct in their range and distindt in 
their effedts. In the first — the Glacier period — the ice, 
moving gradually southward, scored and polished the rocks 
over which it passed, and left behind it the unstratified till 
containing principally scratched fragments of local rocks. 
In the other — the Iceberg period — rocks were carried many 
miles beyond the limit that the glacier ice reached to, and were 
dropped on the top of loose unconsolidated clays and sands, 
which show no trace of any abrading or disturbing force. 
In Europe the ice from the Scandinavian mountains reached 
to the southern side of the Baltic, and for the whole distance 
the bed-rocks are glaciated ; but beyond this the iceberg 
drift is scattered for hundreds of miles, and extends to the 
flanks of the mountain-chains that bound the German plain 
to the south ; and that icebergs do not, as a rule, glaciate 
the beds over which they pass may be gathered from this, — 
that as soon as the boundary is left behind to which the 
land-ice undoubtedly reached no more glaciated rock- 
surfaces are seen ; not even on the hills on which the ice- 
bergs must have grounded, as they have left there the 
greater part of the rocky burden they carried. 
The agency of floating ice in the distribution of boulders 
was early recognised by geologists ; but when, later on, 
Agassiz proved that land-ice had also played a most im- 
portant part, if was not clearly perceived that both agencies 
were required to interpret the phenomena, and to this day 
the till — the product of the land-ice — is often confounded 
with the boulder clay, the product of the floating ice. In 
no other department of geology is far-travelled experience 
more necessary than in the study of the glacial beds. The 
knowledge to be acquired in a single province, or even in a 
single country, is not sufficient, for it will be well nigh im- 
possible from that alone to separate what is particular and 
local from what is widespread and general. To limited 
experience I cannot help believing is due the obscurity to 
be observed in many of the memoirs dealing with glacial 
problems. One authority, who has perhaps lived amongst 
northern mountains, ascribes everything to the ahtion of 
glaciers ; another, whose home, maybe, has been on 
southern plains, sees nothing but the agency of water and 
floating ice. 
In studying the glacial beds of North-eastern America 
we must seek to give their proper importance both to 
