Bevieiv and Comment . 
343 
same views but suggests in a foot note (p. 401) that preglacial 
weathering may have formed the basins before the ice came, so 
that all the ice had to do was to sweep out the decomposed 
rock. 
In the latest edition of the same work he reiterates his former 
views which are essentially those of Ramsay, but seems to open 
the door of possibility a little wider than before for the chance 
entrance of other modes of origin for the rock basins. 
G-eikie would seem to belong rather to the conservative class 
on the question of glacial erosion, but he is still claimed by the 
Ramsay school, as a supporter of their views. 
Prof. J. D. Dana b affirms the power of the ice to break off 
rock masses and thus to make boulders and gravel in large 
quantities for transportation. He thought the gravel and 
boulders were incorporated into the ice mostly in the lower one 
thousand feet and largely in the lower five hundred feet. 
He suggests that the fjords of Maine may have been begun 
before the ice age. 
In his Manual of Geology he accepts the possibility of the 
formation of lake basins in soft strata, and thinks the ice would 
rise on encountering harder strata, thus forming the rim of the 
basin. 
Le Conte * 4 believes that all the Sierra lakes that he has seen 
are of glacial origin, with the possible exception of lake Tahoe. 
He has been led to modify his views on this question by ob¬ 
servations in the Sierras. 
He concludes that a glacier by its enormous pressure, and re¬ 
sistless onward movement is continually breaking off large 
blocks from its bed and bounding walls. Its erosion is not 
only a grinding and scoring but also a crushing and breaking. 
It makes not only rock meal but rock chips. It is constantly 
breaking off blocks and making angular surfaces, and then grind¬ 
ing off the angles both of the fragments and the bed, thus form¬ 
ing rounded boulders and moutoneed surface. Its erosion is a 
constant alternation of rough-hewing and planing. Which of 
these processes predominates depends on the thickness of the 
8 Am. Jour. Science, vol. 5, p. 200,1873. 
4 Am. Jour. Science, vol. 10, p. 126 et seq., 1874. 
