INTRODUCTORY. 267 



tively easy, so that already a whole library of them is at hand, the account- 

 ing for these — even when they are of the simplest kind — is in almost 

 every case a matter of extreme difficulty. We can, at best, but hint at 

 what may have been the possible conditions under which the various geo- 

 logical changes occurred ; and when we ask ourselves whether we fully com- 

 prehend the course of events during any, even a very limited, period of the 

 earth's liistory, we shall, if we are honest, find ourselves obliged to confess 

 that we do not. The more recent the geological epoch to the study of 

 which we apply ourselves, the more we are justified in indulging a hope of 

 arriving at a solution of the theoretical difficulties which present themselves ; 

 because the analogy with that which is now taking place will be more 

 complete the nearer the phenomena with which we have to deal aie to the 

 present time. Hence it is that " surface geology " has become of late such 

 an attractive subject for the geologist. It seems as if in that which is most 

 recent, and most acces.sible to investigation, we Lad some chance of reaching 

 theoretical conclusions of value. Even here, liowever, the difficulties which 

 present themselves become but too obvious when we consider how little 

 imaniniity of opinion there is among those who have of late years occupied 

 themselves most busily with this department of geological investigation. 



In endeavoring to throw liglit on the subject before us, it will be found 

 advisable to adopt the following order in the development of the subject. 

 In the first place the present distribution of snow and ice throughout the 

 • world will be briefly considered and described, with so much detail as may 

 be desirable to enable us to form a correct idea of the physical conditions 

 under which precipitated moisture takes the form of snow ; how and where 

 it afterwards becomes converted into ice ; and Avhy this ice accumulates in 

 certain regions in much larger quantities than in others seemingly not very 

 diffijrentlv situated. 



The next step will be to show, by a statement of what has happened 

 daring the past few centuries, even within fifty years, how greatly the 

 dimensions of existing glaciers ma}' vary within a brief space of time, while 

 we are powerless to detect any corresponding climatic changes with which 

 the fluctuations of the ice misht be connected. This will enable us to com- 

 prehend the possibility of a former much greater extension of snow and 

 ice in certain regions, without the necessity of invoking the aid of violent 

 changes of climate, such as have been deuianded by the theories most in 

 vogue among geologists. 



