Shaler.] 146 [November 4, 



In the preceding comimmication. Mr. Dall has called attention to 

 the fact that throughout the valley of the Yukon River, and the 

 regions lying to the northward, there is a marked absence of all those 

 indications which, in corresponding latitudes in other portions of the 

 northern and southern hemispheres, afford such unquestionable evi- 

 dence of extensive glaciation during the preceding geological period. 

 He assures us (and we cannot but believe that his evidently painstak- 

 ing observations are quite trustworthy) that he failed, with careful 

 search, to find a trace of glacial striation, or any such accumulations 

 of gravel and boulders as characterize the southern portion of the 

 eastern half of the glaciated area of this continent. There seems the 

 more probability of the correctness of the observations of this ener- 

 getic explorer, when we consider the tact that a number of geolo- 

 gists have denied the existence of all glacial deposits throughout 

 Siberia; and the merit of these observers is so great that it would 

 seem that we must accept the want of these evidences throughout this 

 great area as one of the facts to be explained by any theory of the 

 nature and cause of glacial periods. Mr. Dall's observations, should 

 they prove to be well founded, will only compel us to conclude that 

 the region of scanty evidence of glaciation was not confined to the 

 Asiatic continent, but extended under the same pai'allels into North 

 America. 



The existence of these reraai'kable apparent exceptions to the con- 

 tinental extension of the ice sheet during the glacial period, makes it 

 the duty of the geologist to consider, with much care, the nature of 

 the evidences of glacial action, and to determine how far the theory 

 of the conditions existing during that period requires to be modified 

 to suit these seeming exceptions. 



Within four years the theory of the glacial period has made impor- 

 tant advances. The insufficient hypothesis which refen-ed the whole 

 of the changes of climate to alterations in the distribution of sea 

 and land, together with variations in the height of the continents, 

 has been partly abandoned; while the old suggestions of Sir John 

 HerschcU, that the change in the eccentricity of the earth's orbit 

 might have a great influence upon the climatic conditions in either 

 hemisphere, has been again called into notice, submitted to careful 

 examination by several competent physicists and mathematicians, 

 and put in such shape as to claim recognition as the only theory, 

 yet devised, which is competent to supply the forces required to ac- 

 count for the glacial period. 



Accepting this theory, we are at once provided with better means 



