CAUSES OF THE COLD OF THE ICE PERIOD. 281 



and investigation, to determine if possible the causes of these remark- 

 able changes of climate. 



Up to the present time, however, no theory has been proposed 

 which has been sustained by really satisfying evidence, and there is 

 still much difference of opinion on the question among those who 

 know most about it. 



As the subject is one of peculiar geological significance, and great 

 dramatic interest, I venture to bring forward some notes upon it, taken 

 from the geologist's standpoint, hoping that they may contribute in 

 some slight degree to the solution of the problem. 



The theories which have been pi-oposed to account for the cold of 

 the Ice period divide themselves into two groups, viz., the cosmical 

 and terrestrial ; or those which invoke extraneous or astronomical in- 

 fluences, and those which look to changes in the earth itself, or on its 

 surface, for a sufficient cause or causes. 



In the first category may be enumerated the theory of Prof. Croll, 

 that variations in the eccentricity of the earth's orbit have induced 

 great alternations of climate on portions of the earth's surface ; that of 

 Belt and Drayson, which supposes the known variability of the angle 

 of the pole with the ecliptic to have been at times sufficiently great 

 to have brought arctic conditions locally down into the temperate 

 zone; also, the speculations that the heat evolved from the sun has 

 been variable in quantity, that the earth has at various times passed 

 through cold spaces in the universe, etc. 



In the second category are the views first put forth by Lyell, ac- 

 cording to which all the variations of climate recorded in geological 

 history have been induced by changes in the earth itself or on its 

 surface. 



In this paper I shall consider only the latter theory, leaving the 

 discussion of the astronomical aspects of the subject to astronomers, 

 mathematicians, and physicists, who alone are competent to thor- 

 oughly investigate them. 



The explanation given by Lyell of the cold of the Ice period is in 

 conformity with his characteristic conservatism. It is well known 

 that the climatic conditions of all parts of the earth's surface are pro- 

 foundly affected by their topographical features. This may be seen 

 at a glance by reference to any map on which the isothermal lines are 

 delineated. Continental surfaces are known to be productive of ex- 

 tremes of temperature, while the climate of sea areas is comparatively 

 equable ; and the general character of the climate of land and water 

 surfaces is further and locally affected by the configuration and alti- 

 tude of the land, by the breadth and depth of the oceanic basins, and 

 especially by the ocean-currents. The sea forms the great evaporating 

 surface, and the source from which is derived the enormous quantity 

 of water transported by the system of atmospheric circulation. The 

 local climate of continents is also largely influenced by the winds 



