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POPULAR SCIENCE REVIEW. 
their depth to a moraine dam. That is to say, the rock basin is 
imperfect on one side, and there an old glacial moraine may 
have helped to dam the waters back ever since the retreat of the 
glacier which threw off the moraine. It frequently happens 
that a little moraine material has been left upon ice-rounded 
rocks at the foot of a tarn, and in such cases a hasty observation 
might lead one to believe that the whole mound was a moraine. 
Let us remember, then, that a tarn may lie in a complete rock 
basin, ice-formed ; in a glaciated hollow dammed on the lower 
side by a moraine or other accumulation of rocky debris ; or it 
may owe part of its depth to a rock-enclosed hollow, and part to 
a morainic dam. Therefore, on a summer’s day, as we lie 
dreamily gazing upon the rippling waters of these mountain 
tarns, we may sometimes think of an age : which is past, when the 
ice-sheet moved majestically over the now heather-clad fells, and 
all the country lay “clad in white samite, mystic, wonderful.” 
