5$ PEOCEEDIK'GS OY THE GEOLOGICAL SOCIETY. 



indulgence I intend to avail myself hy selecting petrology*, because 

 of late years I have more especially devoted myself to investigating 

 that branch of geology. In so doing I cannot hope to command 

 the sympathies of more than a limited portion of my audience ; but 

 I venture to think that it may be of use to state, as clearly as may 

 be in my power, some of the results at "which I think petrologists 

 have arrived, and some of the difficulties which yet remain for solu- 

 tion. It will not, however, be my purpose to endeavour to ascer- 

 tain whether the former category commands a majority of votes 

 among geologists, or even among nominal petrologists ; I shall 

 venture to speak, as it has been my habit to work, with considerable 

 independence, and you must receive my remarks as mainly the ex- 

 pression of my own opinion, which, however, I can assure you, in 

 all matters still the subject of controversy, has not been formed 

 without considerable thought and labour. This method of proce- 

 dure has been rendered inevitable by the circumstances of the case. 

 It is now some fourteen years since, owing to a combination of 

 causes, I drifted into studying the microscopic structure of rocks. 

 I say " drifted," because I did not undertake the study deliberately, 

 nay, I refrained from it for some time through fear that a certain 

 delicacy in my eyes would make it impossible for me to work with 

 the microscope. That fear, however, has happily proved to some 

 extent groundless, though I have always had to exercise considerable 

 caution, and to use high powers only for limited times. I began 

 the study in consequence of finding among those who professed to be 

 authorities such a conflict of opinion, even upon the most fundamental 

 questions, that no conclusion, upon written evidence alone, seemed 

 possible. Hence it has always been my endeavour to work without 

 prejudice in favour of any view, to gather my facts from observa- 

 tions both in the field and with the microscope, to frame hypotheses 

 as the facts accumulated, and then to continue to test these hypo- 

 theses by further work. I believe that I may add that, at the 

 beginning, I always took as the more probable hypothesis that which 

 appeared to find acceptance with the best authorities. I have endea- 

 voured, in short, to apply to this branch of geology the laws of 

 reasoning which were taught me years ago in mathematics, and I 

 venture to believe that this method is the safest. It will not save 

 one from mistakes ; it may happen that, notwithstanding aU care, 

 one generalizes too hastily ; a wider knowledge may show that an 

 hypothesis is inadequate or imperfectly stated ; but still, I believe 

 that this method of successive approximations, of framing working 

 hypotheses from facts, and constantly exposing these to the test of 

 new facts, is the right way to attain truth in science. 



Let me, however, first venture one or two remarks on the present 

 position of petrology as a branch of science. It has undergone 

 vicissitudes. To many of the early geologists it presented great 



* Or, as some authors prefer to call it, petrography. The word used in the 

 text appears to me preferable on the analogy of the other designations of 

 sciences, petrography indicating the description rather than the scientific 

 investigation of rocks. 



