182 SPARKS FROM A GEOLOGIST'S HAMMER. 



ern elevation throughout all the arctic and sub-arctic 

 regions. It is indeed manifest that the northern regions 

 have undergone great changes of level. All the United 

 States north of the Ohio, and all Canada, have stood at 

 a lower level since the present surface was finished; and 

 there is ground for the belief that just before this subsi- 

 dence they stood at a higher level than at present. But 

 these oscillations can hardly be conceived an adequate 

 cause of continental glaciation. They do not seem to 

 possess the requisite efficiency; nor have they been timed 

 to suit the relations of causal antecedence to the great 

 phenomenon. It is more probable that the elevation which 

 has taken place is to be regarded as an incident or effect 

 of general glaciation, rather than the cause of it. 



Finally, scientists have turned their attention again to 

 the search after an astronomical cause of the great ice- 

 age. That the cause was astronomical seems indicated by 

 the proofs of a succession of ice-ages. Astronomical move- 

 ments describe great cycles. At the end of a certain 

 period the old conditions are reproduced, and the old 

 results are reenacted. The principal ones of these astro- 

 nomical causes I shall attempt to explain in outline, es- 

 pecially that based on variations in terrestrial eccentricity. 

 The subject, however, will demand the thoughtful attention 

 of the reader. It is a subject not always rationally com- 

 prehended, even by geologists who accept the authority for 

 an astronomical origin of ice-periods. 



There are three values in connection with the earth's 

 movements, changes in which must affect the earth's cli- 

 mates to some extent. These values are: 1. The inclina- 

 tion of the earth's axis to the plane of the ecliptic; 2. 

 The precession of the equinoxes, or position of the peri- 



