Geology and Natural History. 817 
nary movements of the crust,—the upward for the Glacial period, 
the downward for the Champlain or Fluvial period, and then a suc- 
ceeding wpward rise—like the other great movements, to the lateral 
pressure in the earth’s crust due to contraction from cooling. 
Adhémar’s hypothesis, as Shakya by Mr. Shaler, supposes a dis- 
placement of the center of gravity in consequence ‘of the unequal 
accumulation of ice at the poles, much like that recently proposed 
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oes not increase with regularity over the whole northe eml- 
sphere; and then presents his own hypothesis, which attributes the 
depression to the weight of the ice-mass over the land. er 
speaking of the continents as having their positions determined 
by “constant tensions” (meaning, aie to include lateral pres- 
sure, which far ramet tensions in importance) and the bin 
the mass, he says, “it seems seidiait enough that we may m 
reasonably look to ie weight of the ice accumulated on the Soues 
nents during the Glacial period for the depression of the land-areas 
it A than to any ee cause.” 
his theory assumes that of ice “a mile or more thick,” 
equivale sat as the author states to half this thickness in “ ordinary 
rock, = depressed the eek, erust 500 feet an oe ward over the 
by Mallet t to be orn ous, it seems to be ed from proba ble that 
mere weight can account for s great a movement. Lateral pres- 
sure Se been the chie agent it in Cauneeouk: of level and mountain- 
ng over the earth; since Archean time, making (1) the Green 
Modsiaine: (2) ridges in ee see Scotia and New Brunswick, and (3) 
the Alleghanies, on the eastern border of the North American 
Continent, at lo servis : at subsequently, after pes te in- 
tervals i i a Ne- 
nd ( 
on the western side of the Continent ; bondch (4) elevating, during 
the 5 ates d the great Rocky Moun tain mass 8,000 to 10,000 a 
ally ahothice consequence of the uneasiness of the crust owing to 
lateral pressure 
