Notices of Memoirs—Dr, M. Manson on Glaciations. 423 
Pleistocene time as the most significant transition epoch of the climatic 
history of the earth. 
There seems to be a tendency in recent years to fall back upon the 
hypothesis of variations in the emissive power of the sun to account 
for the variations of surface temperatures indicated by ice formations.* 
It is certainly quite possible and even probable that the sun’s 
emission has altered within geological time, but neither the distribu- 
tion of fossil life nor the evidences of ice formations occur with that 
zonal arrangement in harmony with solar-controlled climates, so that 
to the writer it seems necessary to attribute geological climates to 
a uniformly distributed source of heat, and to eliminate solar control 
by the reasonable assumption of persistent cloudiness. It is not 
implied that the clouds were so thick as to prevent the transmission 
of light such as is now received on overcast days, but a far less thick 
layer than that would suffice to screen off most of the solar-heating 
effects. Moreover, under the hypothesis that solar radiation was 
interrupted by a thin but continuous stratum of cloud, there is no 
reason why glaciers should not flow into the sea at any latitude. 
When cloud densities decreased to approximately present conditions 
the tropical zones of downcast currents and minimum cloudiness were 
the regions first affected, and in these solar radiation first reached the 
surface. ‘hus solar radiation which, with a continuonsly cloudy sky, 
fixed the tropical zones as regions most exposed to cold downcast 
currents and to glaciation, also fixes them, with present cloudiness of 
52 per cent. of the earth’s surface, as regions of maximum exposure 
to solar radiation. The great continental glaciers of the northern 
hemisphere were grouped about the North Polar region for the reason 
that continents are so grouped, and for the additional reason that 
atmospheric circulation fixes latitude 50° N. as one of the belts of 
maximum storm circulation and precipitation. 
Solar climatic control distinctly manifests itself by a zonal arrange- 
ment of temperatures and of life; under this control the disappearance 
of Pleistocene ice-sheets is taking place. ‘The earth is therefore not 
in an era of senility or decrepitude, but in the springtime of a new 
life in which nobler, higher types of life are being developed. 
The difficulties attendant upon the previous explanation of climatic 
phenomena appear to the author to be due to false premises, namely, 
(1) that solar radiation controlled the climates of Pleistocene and 
previous eras; and (2) that effective earth heat, under the extremely 
slow processes of loss and bringing into effectiveness and the powerful 
processes of conservation, was entirely lost prior to the era of zonally 
disposed climates. Upon a rejection of these assumed premises we 
may freely admit that ice has been a geologic agent from the earliest 
ages, particularly upon land masses of low specific heat and extremely 
low conductivity, and that the regions of cold downcast currents were, 
prior to the Pleistocene, most exposed to cold downcast currents and 
to consequent local glaciation; and that as the supply of earth heat 
fluctuated, ice formed under favourable conditions over large geographic 
areas in any latitude and during many eras, to disappear from an 
1 Professor David, Trans, Tenth Int. Geol. Cong., Mexico, vol. i, pp. 481-2, 1906. 
