RVE[J)PMKNT OF PETROLlKiY \)V\\m\ THE 

 PRESENT CENTdliY. 



(Prksidential Address). 



By HENRY C. RICHARDS, M.Sc, 



Lecturer in Geology, University of Queensland. 



Bead at the Annual Meeting of the Royal Society 

 of Queensland, ^Oth March, 1914. 



In 1857 the late Dr. H. C. Sorby coniinunicated to the 

 Quart. Journ. Geol. Soc. two very important papers em- 

 bodying the results of his investigations on the micro- 

 scopical structure of crystals, with particular reference to 

 the origin of minerals and rocks. Previous to this several 

 other investigators — (brdier, Bunsen, Durocher, Scrope, etc., 

 had been investigating the origin of rocks, but the impetus 

 of Sorby's work was ]3articularly pronounced and has had 

 a very great influence. 



Many factors have played a prominent part in the 

 recent advances in Petrology, but the development in physi- 

 •cal-chemistry, the more intimate knowledge of the structure 

 of the earth's crust, the accumulated knowledge of the dis- 

 tribution of rocks both in space and time, the perfection 

 of optical determinations, and a more intimate knowledge 

 of the specific properties of minerals, have very considerably 

 furthered our knowledge of the origin of rocks, their diversity 

 and genetic relationships. 



In petrogenesis the dominating factor at the close 

 •of the last century was, perhaps, differentiation, but the 

 application of the principles of solution to the crystalliza- 

 tion of igneous rock-magmas by Vogt and his determina- 

 tion of the approximate " eutectic ratios " for a number 

 of pairs of minerals accounted for the theory of eutectics 

 playing the chief role for a few years. 



The principles of differentiation, eutectics and the 

 .absorption and assimilation of rock-matter by molten rock- 



