1/4 J^^'-^ American Geologist. Feptembcr, li^tn 
white area disappears as the pole is turned towards the sun in 
the following summer. The disappearance on the edges is 
rapid, but the true polar areas are persistent for months. They 
are evidently composed of that constituent of the atmosphere 
of the planet which has the highest freezing point, for upon 
their disappearance we observe the surface of the planet.''' 
These phenomena admit of the simple explanation usually 
rendered, namely, that these areas. are polar snow-caps which 
form and melt ofif with the seasons ; but the task of explaining 
how a planet receiving less than half the heat and light which 
the earth receives could enjoy so mild a winter and so warm a 
summer at polar latitudes has by some been considered diffi- 
cult. 
TJic Omitted Factors. 
In the life of a planet we have seen that there may be a 
period between the effective exhaustion of its own available 
heat and the reign of solar heat, during which period glacial 
conditions are extensive for a great length of time. This period 
the earth has manifestly passed through, as continental glaciers 
once existing have disappeared, and the isolated glaciers are 
yet retreating upwards and polewards in tropical, temperate, 
and even polar latitudes. 
It has been stated that the actual amount of heat received 
by a planet was not the sole factor influencing its surface tem- 
peratures; other factors are equally and perhaps more impor- 
tant. Their existence and influence are made apparent in the 
general rise in terrestrial temperatures since the culmination 
of the period of great glacial extension over now temperate 
and tropical areas. One of these factors or causes is the power 
of the atmosphere to trap or selectively absorb solar energy. 
*It has been argued by some who think that because Mars receives 
less than .43 of the solar heat which the earth receives that its climates 
must necessarily be much colder than ours; and that these polar seas 
may be congealed carbon dioxide, (CO2) which has a freezing point 
of -109° Fahr. (See Publications of the Astronomical Society of the 
Pacific, Vol. VI, No. 38. pp. 380, 300, 302. Also Vol. VII, pp. 54-55-) 
To the writer this idea is, however, untenable, for before the low 
degree of 109° below zero Fahr. could have been reached, all vapor 
of water and other volatile constituents of the atmosphere of Mars, 
congealing at a higher temperature than -109° Fahr. would have con- 
densed, and would permanently shroud the planet in white. Upon 
this surface the deposition and melting off of the equally white carbon 
dioxide could not be observed. 
