94 T^he A?nerica7i Geologist. August, i899 
distance and surroundings have varied the rate of development 
of each. At any particular period progress has not been the 
same for all, and we find each one in that particular stage 
which its mass, environment and exposure have permitted it to 
reach ; yet all are governed by the same general laws and must 
be progressing toward the same general condition.* 
The first problem in the series has attracted not only the 
geologist, but the astronomer. The mathematician, undis- 
mayed by lack of data or lack of a fulcrum on which to resi his 
powerful analytical lever, and aided mainly by hypothesis and 
assumption, has striven to limit or extend the lines of study. 
Awed by the intricacy and apparent accuracy of his calcula- 
tions, and fearing to attack his data, others have been led into 
error; and thus the attempts to apply the most accurate branch 
of science to these questions have occasionally thwarted the 
advance of knowledge. 
The problem is a complicated one; in its solution the laws 
of physics must be rigidly followed and the geological record 
of the climatic changes which the earth has passed through 
must be correctly interpreted. The conditions which admit of 
the seasonal melting ofif of the polar snow caps of Mars on the 
one hand, and the clouded atmosphere of Jupiter on the other, 
must yield confirmatory evidence. 
Fortunately the elementary principles, the premises upon 
which the argument must be based are fundamental in their 
character and are generally admitted to be true. But before 
going further it is necessary to clear the field of certain fal- 
lacies. This can best be done by a short review of the principal 
attempts which have been made to solve the problem, and by 
a presentation of the conclusions which the master minds of 
geology have reached. 
*This, so far as the author has been enabled to investigate, is not 
true of satellites; for they seem to have been deprived of all volatile 
matter, and therefore have been and are exposed to very different con- 
ditions of radiation and reception of heat, as compared with planets. 
The author believes that it is demonstrable that if a planetary mass 
equivalent to the combined masses of the earth and the moon be differ- 
entiated by any process into the two existing masses, the larger mass 
must retain all volatile matter; also that where the attraction between 
two masses is sufificiently great to turn the smaller mass with its denser 
portion toward the larger and hold it in this position without inde- 
pendent axial revolution, the larger mass will attract and retain all 
volatile or gaseous matter. 
