Harkcr : Petrology in Yorkshire. yi 



and we do not know which ti^•e, it is clear that the doubt must 

 ■extend to the whole list. 



Regarded merely as specimens gathered from many sources, 

 without reference to the problems of glacial geology, our 

 boulders provide a rich store of material for the student of 

 igneous rocks. Not to mention such areas as the Lake District 

 and the Cheviot Hills, there is the wonderful series of rocks 

 of the Christiania district, without parallel in our own country. 

 Not a few of the interesting types made known to petrologists 

 chiefly by Professor Brogger have already been recognised 

 among the boulders of Holderness and the coast, and it cannot 

 be doubted that many others will reward a search aided by 

 microscopical study. The preparation of thin sections is 

 now greatly simplified and made easy, and there is no reason 

 why everybody should not make his own sections, as Sorby 

 did fifty years ago, but with much less expenditure of time and 

 labour. A cutting-lathe is not requisite, nor indeed does it 

 now afford any advantage ; for, thanks to the use of carborun- 

 dum as an abrasive, it is less troublesome to grind down a chip 

 than to cut and reduce a slice. The work of making and 

 mounting thin sections of any kind of rocks is thus easily 

 performed at home, and at a trifling cost for material and 

 appliances. Moreover, this procedure has positive advantages. 

 It is possible to ensure that the section shall include precisely 

 the desired part of the specimen, and sometimes useful in- 

 formation may be gained during .the operation of grinding. 

 In some cases, too, it is convenient to be able to treat the section 

 with some reagent before adding the cover-slip. Thus, such 

 minerals as nepheline and analcime may be made much more 

 apparent by attacking the surface with h5/drochloric acid and 

 then (after washing) staining the gelatinous silica so produced 

 with fuchsine or some other appropriate colouring medium. 



It is, I fear, impossible that an address on a special subject, 

 offered to a general audience of naturalists, should succeed 

 in interesting all. My immediate predecessor, my friend 

 Professor Seward, could appeal to two sections at least, the 

 botanists and the geologists, but in this versatility I am not 

 qualified to emulate him. To my non-geological hearers I 

 can only proffer the consolation that their turn will come iii due 

 course another year. To the geologists I make no apology 

 for confining the tenour of my remarks to a single branch, 

 inasmuch as my chief object has been to urge the claims of 

 Petrology to a fuller measure of consideration than it has yet 

 received. I have shown it, not as an isolated study, but as an 

 integral part of geological science, having intimate relations 

 with historical geology, physiography, glacial geology, and 

 numerous other lines of inqulr3^ 



I wish to insist upon this aspect of the matter, because I 

 think it rests mainly with amateur workers to preserve that 



1912 Mar. I. 



