Harker : Petrology in Yorkshire. 41 



Sorby ; for we must remember, not only that modern petrology 

 had its birth in Yorkshire, but also that it was in the beginning 

 the petrology chiefly of sedimentary rocks. It is true that 

 interest has since been diverted more especially to igneous 

 rocks and crystalline schists ; but this certainly does not imply 

 that the earlier field of research is exhausted, nor that, in the 

 problems which it still presents, it is inferior in interest to the 

 more trodden paths. Indeed, its possibilities have been made 

 sufficiently evident by other workers in recent years, although 

 in his own country of Yorkshire the prophet is, I think, still 

 without the honour which he would most have appreciated — 

 that of imitation. 



It should be remarked that in the study of sedimentary 

 deposits, the petrologist of to-day has at command simple and 

 easily applied methods of working, which were unknown to the 

 pioneer in this line of research. This will be best illustrated 

 by one or two examples. Some of Dr. Sorby's earliest in- 

 vestigations were concerned with sands and sandstones as 

 throwing light on the physical geography and geology of 

 former periods, especially as regards the direction from which 

 the deti'itus was derived and the nature of the parent rock- 

 masses which furnished it. He paid particular attention to 

 peculiarities of bedding due to currents of different velocities 

 in water of different depths, a subject on which the last word 

 has not yet been said ; but he also obtained interesting results 

 from a minute examination of the sand-grains themselves. 

 This latter line of study has been shown to be capable of 

 considerable development. Significance attaches not so much 

 to the grains of quartz and felspar, which make up the chief 

 bulk of most deposits of this kind, as to the rarer constituents. 

 These existed originally as minor accessory elements in the 

 igneous and crystalline rocks from which the material has 

 been directly or indirectly derived, and different groups of 

 crystalline rocks have their own characteristic accessory 

 minerals. Thus, a certain type of granite will yield fragments 

 of tourmaline ; many igneous rocks of more basic composition 

 furnish abundant grains of magnetite and ilmenite ; crystalline 

 schists representing old metamorphosed sediments will give, 

 in different cases, andalusite or cyanite or staurolite or garnet ; 

 and so for other types of rocks. These minerals are recog- 

 nisable in small fragments, and afford a valuable clue in 

 tracking the sediment to its source. They occur, however, 

 in most cases only sparingly amidst a great preponderance of 

 quartz-grains and other less significant constituents. From 

 these the accessory minerals may be separated by taking 

 advantage of the fact that they are all of comparatively high 

 specific gravity. Most of the lighter minerals, such as quartz, 

 can be washed away with water, the most convenient utensil 



1912 Feb. I. 



