6o PEOCEEDDrGS OF THE GEOLOGICAL SOCIETY. 



observed that the opinion of one who is known to have devoted some 

 years to the study is considered to outweigh that of one who has 

 given DO proofs of competency ; while I have seen in petrology, again 

 and again, statements commanding attention, and even printed in 

 scientific journals, which I knew not only to be unproved, but also, 

 unless the work of years had been wasted, to be extremely difficult 

 to prove. 



There is yet another impediment to progress, quite opposite to, 

 and much more laudable than, the last. It is that the recognition 

 of the great difficulties of the subject causes some students to despair 

 of arriving at the truth, and leads them to adopt what I may call 

 an agnostic position. J^ow I believe that by so doing no progress 

 ever has been or ever will be made in science. If caught in a 

 scientific " slough of despond," you will never get out by merely 

 wallowing about aimlessly. Assume that there is a way out ; try 

 in turn each that seems most probable, and probably one will be 

 found. I am well aware that there are a vast number of questions 

 in petrology which are not yet settled — not a few, perhaps, never 

 will be settled in our lifetime ; but I maintain that progress is most 

 likely to be made by endeavouring to frame a working hypothesis, 

 if only the observer be strictly honest in recording not only the 

 facts which are favourable to it, but also those which appear to be 

 hostile. May I then be allowed, before proceeding further, to lay 

 down two principles which occasionally seem to be forgotten by some 

 earnest workers ? 



{a) The first is that aU observations which are on record are not 

 of equal value. This depends, as I have said, partly on the qualifi- 

 cations of the observer, partly on the perfection of his appliances. 

 For example, there are some difficult points in petrology in regard 

 to which I should attach hardly any weight to the evidence of even 

 one of our most honoured workers in " premicroscopic " days, because 

 I know, from my own experience, that the question is one where 

 the unaided eye cannot help us to a decision. 



(5) That one "positive" observation outweighs a large number 

 of " negative," the latter word being used in the sense of " leading 

 to no definite conclusion." Let me illustrate this by an example. 

 Suppose I wanted to ascertain whether an igneous rock were inter- 

 bedded with or intrusive among certain sedimentary strata, and 

 that twelve sections were to be found. Suppose that in eleven of 

 these the appearances were not inconsistent with either interpre- 

 tation, but that at the twelfth the evidence could only lead to one 

 of the two views. Clearly the absence of conclusive evidence in the 

 eleven cases would not affect the value of the evidence in the 

 twelfth. 



The rules, in short, of ordinary reasoning — and this remark has a 

 wider application than to the case which I have just mentioned — hold 

 in every branch of science, and in no one is it more needful to bear 

 them in mind than in geology, where direct experiment (as in 

 chemistry and physics) is so often impossible, and where we can 

 never attain to more than a degree of moral certainty. 



