CONDITIONS OF PAST GLACIATION DISCUSSED. 343 



has been bea;un, beeauso the ice which once threatened his cultivated fields 

 has now withdrawn into the recesses of the mountains. It is the case, how- 

 ever, that geologists have in certain regions, as will be seen farther on, been 

 too much inclined to magnify the importance of local changes of condition, 

 limited perhaps to a few square miles of their own immediate vicinity, and 

 to look upon some trifling difference between the two successive members of 

 a group of stratified detrital material as indicative of world-wide changes. 

 All local details of structure are of value and importance from the local point 

 of view; but it is absolutely necessary in geology to generalize from a mass 

 of facts gathered over a wide region, and the theory of the science would 

 make but little progress if the minute modifications of each locality were 

 considered as necessarily to be repeated in exactly the same order, and with 

 similar comparative importance, all over the world. 



In preparing the sketch of the present distribution of ice and snow 

 throughout the earth, given in a preceding section, we found an abundance 

 of material ready to hand to enable us to form, except in the case of the 

 Antarctic Polar region, a pretty clear idea of the locality and extent of the 

 areas thus covered, and of the climatic and topographical conditions there 

 prevailing. The facts are easily obtained, and theoretical difficulties do not 

 present themselves when we arrange and explain them. In endeavoring to 

 make out what regions have been at any former time the permanent abode 

 of ice, where now there is nothing of the kind present, we find that we have 

 a much less easy task before us; for we have only the traces which those 

 formerly existing glacial masses have left behind them by winch to be guided 

 in our work ; and those traces are often of a very uncertain character, not 

 only as originally left upon the surface, but as afterwards having been sub- 

 jected during longer or shorter periods to all the accidents of weathering and 

 erosion. Hence, the geologist who has had the most experience in studying 

 the character of glacial markings cannot always come to a correct decision 

 as to the nature and origin of the surfaces presented to him in his investi- 

 gations, so that mistakes are likely to be made even where the observer has 

 tlie necessary qualifications for his work, and is not hampered in it by preju- 

 dices for or against the various theories which have been started to account 

 for the phenomena of glaciation. Thus the size and position of the areas 

 formei'ly covered by snow and ice are, in spite of all that has been done in 

 this department of geology within the past few years, matters of considerable 

 imcertainty. especially in this country, where much of that which has been 



