THE PAST AND FUTURE OF GEOLOGY. 187 



more temperiite methods of explauatioii ; but it is a question whetber 

 the license which formerly was taken with energy is not now too much 

 taken with time. Small forces long continued, action frequently 

 repeated, and maintained uniformity of operation, are accepted as suffi- 

 cient to account for the formation of our bills and plains, of the Alps 

 and the Andes, and for all the great general as well as special features 

 of the earth's crust. 



I am aware that in expressing other views I shall have occasion to 

 differ from men for whose opinion 1 have the highest regard, and who 

 have done infinite service to the progress of scientific geology ; but I 

 am also expressing views which 1 was very early led to form, and which 

 long experience has only tended to confirm. The points at issue are, 

 firstly, whether our experience on these questions is sufficient to enable 

 us to reason from analogy; and, secondly, whether all former changes" 

 of the earth's surface are to be explained by the agency of forces alike 

 in 1:1)1(1 and degree with those now in action. It is not possible in the 

 limits of this address to do full justice to these important questions. I 

 may, however, briefly state my reasons for answering these questions in 

 the negative. 



The value of experience with respect to natural phenomena depends 

 upon whether they are symmetrical and not variable, or whether they 

 are variable and unsymmetrical. In the one case, as any one part bears 

 a given uniform relation to the whole, if one part be known, the whole 

 can be inferred; but in the other case, whei^e the whole is made up of 

 unequal and not uniform parts, the value of the evidence is merely in 

 proportion to the number of those parts independently determined, or 

 to the ratio between the duration of the observation and the duration 

 of the time comprising all the phases of the particular phenomenon. 

 Thus the path of a planet, the date of an eclipse, or the return of a 

 comet, may be predicted with certainty by the determination of mere 

 minute sections of their orbits, which in respect to time are infinitely 

 small compared to the length of the cycle of revolution. On the other 

 Land, the metamorphosis of an insect, the mean temperature of a i)lace, 

 or the character of a volcano, can only be accurately determined by a 

 length of observations sufficient to embrace all the variations they resj^ect- 

 ively present in their several cycles of change. In the case of the insect, 

 the time must be equal to the duration of the metanioriAiosis ; in that 

 of temperature, a succession of years is needed to obtain a mean ; and, 

 with respect to volcanoes, centuries may often i)ass before we become 

 acquainted with all the irregular exhibitions of their spasmodic activity. 



The necessity for a much greater extension of time becomes yet more 

 imperative when we come to deal with geological phenoniena, such as 

 those due to the action of elevatory forces, Avhich are extremely varied 

 in their nature, being at one time exhibited by a raised beach, a lew feet 

 high, and at another by a mountain-chain, whose height is measured by 

 miles; or by the small displacement produced by an earthquake, and 



