THE FORMER EXTENSION OF GLACIERS. 1 47 
doubt that they have been carried by glaciers to their 
present position. 
These great blocks, however, imposing as they 
are, are yet as nothing to the mass of gravel, sand, 
and mud brought down by the glaciers, carried over 
intervening ridges and across lakes, and spread over 
the whole of Switzerland. 
The erratic blocks are unfortunately rapidly dis- 
appearing, as they are much in demand for building 
and other purposes. Some of the most remarkable 
have, however, happily been secured, and will be pre- 
served by the Swiss Scientific Societies. 
Considering the immense magnitude of the mo- 
raines and the enormous number of erratic blocks, it 
is evident that the glacial period must have been of 
very long duration. 
Polished and Striated Surfaces. — Roches 
Moutonnees. 
A third class of evidence is that furnished by 
polished and scratched rock surfaces, which of course 
are best preserved when the material is hardest. 
The rocks are sometimes polished like a looking- 
glass. Such surfaces occur under and round exist- 
ing glaciers, where there can be no doubt that they 
are the work of the ice, or rather of the stones con- 
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