1 1 2 Tlie America?i Geologist. February, i90o 
any hypothesis that assumes to account for glacial periods 
must take serious cognizance of these and must meet the 
strenuous issues that arise from their early age and from 
the mildness of the periods following them." 
The much discussed question of tropical glaciation also 
comes up here. When the isothermal spheroid 32° F. 
shrunk in upon the earth, its intersections with the surface 
were controlled by the elevation of the surface above sea 
level, and by the local escape of earth heat; elevated equa- 
torial or temperature areas were under this interpretation as 
much exposed to glaciation as polar lands. This isotherm 
aJso reached continental areas prior to reaching ocean areas. 
Though the author appears to believe in tropical glacia- 
tion, he says: "Difficulties exist in determining glacial con- 
ditions in the tropics because of the earlier removal of glaci- 
al conditions, the shorter duration of the period of glacia- 
tion, long exposures to the decomposing effects of tropical 
rains and heat, and to the commingling of the classes of 
phenomena." 
It is possible, and perhaps probable, that observers have 
mistaken rock decay and flood deposits for glacial drift. 
Here we may appl)' a rule laid down by the author in his 
article, viz., — that glacial action is not sufficient evidence 
upon which to base the occurrence of a glacial period, but 
there must also be forms of life adapted to live under Arctic 
condition'^-. 
The case against tropical glaciation is well summed up 
b}' Howorth.* "The orchids and palms, the monkeys and 
humming-birds, could not live when ice occupied the Ama- 
zon valley and the latitude of Nicaragua. We cannot un- 
derstand where they could have taken shelter, for it follows 
that if the tropics were cold, the temperate regions must 
have been colder, and we have evidence, in the Brazilian 
caves, that precisely the same genera of animals occupied 
the country when they were filled as now, onh' with larger 
specific forms." 
The theory does not appear to be completel)- worked 
out in regard to interglacial epochs. The incomplete records 
which we have from interglacial beds indicate that these 
*The Glacial Nightmare, p. 491. 
