322 PROF. W. MORRIS DAVIS ON [-A-Ug. 1909, 



supposition concerning the past events of the earth's history can 

 reach general acceptance as a ' fact of inference ' until the conse- 

 quences that follow from it have been critically deduced so that 

 they may be candidly confronted with the ' facts of observation ' : 

 for, until this is done, no safe or impartial judgment can be reached 

 as to the success of the supposition in representing the unobservable 

 facts of the past. 



A paragraph in Plavfair's ' Illustrations ' is here so pertinent 

 that I quote it entire. After pointing out that to avoid all 

 theoretical reasoning 



' would not be caution, but timidity, and an excess of prudence fatal to all 

 philosophical inquiry,' 



this logical writer goes on to say : — 



'The truth, indeed, is, that in physical inquiries, the work of theory and 

 observation must go hand in hand, and ought to be carried on at the same 

 time, more especially if the matter is very complicated, for there the clue of 

 theory is necessary to direct the observer. Though a man may begin to observe 

 without any hypothesis, he cannot continue long without seeing some general 

 conclusion arise; and to this nascent theory it is his business to attend, 

 because, by seeking either to verify or to disprove it, he is led to new experi- 

 ments, or new observations. He is led also to the very experiments and 

 observations that are of the greatest importance, namely to those instanti<& 

 crucis, which are the criteria that naturally present themselves for the trial 

 of every hypothesis. He is conducted to the places where the transitions of 

 nature are most perceptible, and where the absence of former, or the presence 

 of new circumstances, excludes the action of imaginary causes. By this cor- 

 rection of his first opinion, a new approximation is made to the truth ; and by 

 the repetition of the same process, certainty is finally obtained. Thus theory 

 and observation mutually assist one another; and the spirit of system, against 

 which there are so many and such just complaints, appears, nevertheless, as the 

 animating principle of inductive investigation. The business of sound philo- 

 sophy is not to extinguish this spirit, but to restrain and direct its efforts ' 

 (1802, pp. 524-25.) 



XXIII. Confrontation - of the Deduced Consequences of the Two- 

 Theories with the Pacts of Observation in the Snowdon 

 District. 



The most strikingly abnormal forms in the Snowdon district, 

 already briefly described, may now be reviewed in more detail 

 under the titles of valley-heads, valley-floors, valley-sides, and 

 hanging valleys, in order that the consequences of the two theories 

 may be confronted by the facts of form. Choice may then be made 

 of one theory or the other as the more reasonable, according as the 

 consequences of one or of the other best fit the facts. 



Yalley-heads: cwms. — Practically all the higher valley -heads 

 of the Snowdon district are cwms of more or less typical form, with 

 broad floor and steep head and sides. Under the theory that 

 glaciers are protective, we should here find forms characteristic of 

 normal pre-Glacial erosion, except in so far as the removal of pre- 

 Glacial waste and the slight scouring of the underlying rock may 



