370 TRANSACTIONS OF SECTION C. 



Section C— GEOLOGY. 

 President of the Section. — Alfred Harker, M.A., F.R.S. 



THURSDAY, AUGUST 31. 

 The President delivered the following Address : — 



Some Aspects of Modern Petrology. 



In accordance with the custom which permits the occupant of this chair to open 

 the proceedings with observations on some selected subject, I wish to invite your 

 attention to certain points concerning the genetic relations of igneous rocks. The 

 considerations which I shall have to lay before you will be in some measure 

 tentative and incomplete; and indeed, apart from personal shortcomings, this 

 character must necessarily attach to any discussion of the subject which I have 

 chosen. For petrology is at the present time in a state of transition — the 

 transition, namely, from a merely descriptive to an inductive science — and at 

 such a time wide differences of opinion are inevitable. If I should seem to do 

 less than justice to some views which I do not share, I hope this fault will be 

 attributed to the limitations of time and space, not to any intention of abusing 

 the brief authority with which I find myself invested. 



The application of microscopical and special optical methods, initiated some 

 fifty years ago by Dr. Sorby, gave a powerful impetus to the study of the 

 mineral constitution and minute structure of rocks, and has largely determined 

 the course of petrological research since that epoch. For Sorby himself obser- 

 vation was a means to an end. His interest was in the conclusions which he 

 was thus enabled to reach relative to the conditions under which the rocks were 

 formed, and his contributions to this problem will always rank among the classics 

 of geology. The great majority of his followers, however, have been content to 

 record and compare the results of observation without pushing their inquiries 

 further; and indeed the name 'petrography,' often applied to this line of re- 

 search, correctly denotes its purely descriptive nature. A very large body of 

 facts has now been brought together, and may be found, collated and systema- 

 tised by a master-hand, in the monumental work of Eosenbusch. Beyond their 

 intrinsic interest, the results thus placed on record must be of the highest value 

 as furnishing one of the bases upon which may eventually be erected a coherent 

 science of igneous rocks and igneous activity. 



In earnest of this promise, recent years have witnessed a very marked revival 

 of interest in what we must call at present the more speculative aspects of 

 petrology. This manifests itself on the side of the petrographer in a growing 

 disposition to seek a rational interpretation of his observations in the light of 

 known physical principles, and on the side of the field geologist in a more con- 

 stant regard for the distribution, mutual associations, and mode of occurrence of 

 igneous rocks. I will add, as another hopeful sign of the times, a decided 

 rapprochement between the laboratory and the field, too often treated in practice 

 as distinct departments. 



