Boulders Due to Rock Decay. — Upliam. 375 
observations, that about half of the larpjc boulders in the gla- 
cial drift of the rorthern United States may be reasonably at- 
tributed to derivation from preglacial residual rock' masses, 
already weathered into boulders, or precariously sculptured 
into tower-like and craggy forms, in citlicr case very easy to 
be borne away in the drift. Yet this prolific source of gla- 
cial boulders has no attention (or at least no emphasis) be- 
stowed on it in the most elaborate treatise on geology, bv Dana 
and Sir Archibald Geikie, nor in the most valuable general 
works on the Ice age, by professor G. F. Wright and profes- 
sor James Geikie. 
In closing this short paper, I wish to add that a very fruit- 
ful study could be given to the relationship of the preglacial 
boulders with the exceptional profusion of boulders always 
observed in the marginal moraines of the continental drift 
areas, as compared with their less numbers on the smoother 
adjoining tracts of the ordinary till. The conditions of en- 
glacial carriage of boulders, giving their great predominance 
in the moraines, has long seemed to me of high interest, and 
not fully understood. Probably many of the boulders early 
gathered into the basal part of the ice-sheet from the pregla- 
cial weathered rocks were carried hundreds of feet above the 
land and during thousands of years, to be finally stranded, 
when the ice was melted away, in the knolly and hilly marginal 
drift accumulations which were formed wherever the ice bord- 
er paused or temporarily readvanced during its general reces- 
sion, under secular climatic changes, remaining for some ge- 
ologically short time nearly stationary at each of the morainic 
belts . 
