Harkey : Petrology in Yorkshire. 43 



Questions of the same kind, and to be approached in the 

 same way, arise with reference to the transportation, distribu- 

 tion, and deposition of sediments under the conditions existing 

 at the present time. The results of inquiry on these lines will 

 not only be of interest in themselves, but may not improbably 

 have important practical applications. Such a problem 

 is the source of the ' mud ' (which, however, is largely sand) 

 of the Humber estuary. How much of this is brought down 

 by the river from the drainage-basins of Trent and Ouse, and 

 how much is carried up by the tide from the waste of the 

 Holderness cliffs ? This has an obvious bearing on questions 

 of warping, of dredging, and of coast-protection. It is not 

 too much to expect that light might be thrown on the matter 

 by a comparative study of the sediments themselves from the 

 petrographical side. 



The true argillaceous sediments — clays, shales, marls, and 

 the like — offer, I think, a less promising field for those workers 

 to whom time, technical skill, and rather costly instruments 

 are considerations of weight. It is true that the most impor- 

 tant British contributions in this line have come from an 

 amateur petrologist, Mr. Hutchings of Newcastle ; but it is 

 clear that the work presents difficulties not to be surmounted 

 by all. The minute size of the elements in these fine-textured 

 deposits demands the use of high magnifying powers and special 

 modes of illumination, and chemical analysis is almost a 

 necessary concomitant of the microscopical work. 



Nevertheless, apart from research of this order, there is 

 room for simple and easy observations on argillaceous rocks, 

 which, carried out systematically, have their value. The 

 minute elements ,of a clay are not all of the same grade of 

 magnitude, and a partial separation may be effected among 

 them according to the time required for settlement in water. 

 By this fractionating process we obtain a rough mechanical 

 analysis, which is not without significance for purposes of 

 comparison. Most easily isolated is the sandy element which 

 is seldom wanting in ordinary argillaceous deposits ; and in 

 clays, no less than in sands, we may find little crystals of 

 those accessory minerals which I have mentioned as possessing 

 a special importance in the indications which they furnish. 



Turn now to another class of sedimentary rocks, the lime- 

 stones. These offer a specially inviting field to the amateur 

 worker, in that the preparation of thin slices for the microscope 

 is a much easier and more rapid process with limestones than 

 with igneous rocks. I will cite one point of interest, where 

 petrology comes into contact with conchology. It is well 

 known that lime carbonate exists in nature in two distinct 

 forms, calcite and aragonite, and that both may be secreted 

 by living organisms. In his Presidential Address to the 



1912 Feb. I. 



