602 REPOET— 1886. 



Once for all, I ask you to bear in mind that this address is mainly a recital of 

 other men's work, so that I shall not need to interrupt its continuity by remarks 

 as to the original observers. The subject is, indeed, one to which 1 have paid 

 some attention, but I can only call myself a humble follower of such men as God- 

 win-Austen, ' the physical geographer of bygone periods,' and Sorby, who was the 

 first to apply the microscope to similar problems, and to whom in this class of in- 

 vestigation we may apply the well-known saying. Nihil tetigit quod non ornavit. 



With the deepest gratitude also I acknowledge the loan or the gift of specimens 

 from Drs. Hicks and Callaway, from Messrs. Howard Fox, Somervail, Shipman, 

 Gresley, Houghton, Marr, Teall, and J. A. Phillips, from Professors Lapworth and 

 Judd. Through their liberality I have had the opportunity of examining for my- 

 self the greater part of tlie materials which have already been described in the 

 principal geological periodicals, and of adding many new slides to my own 

 collection. 



The nature of the materials of grits and sandstones has been so admirably 

 treated by Dr. Sorby in his presidential address to the Geological Society for 1881 

 that I may pass briefly over this part of the subject. I will, however, add a few 

 details in the hope of placing more clearly before you the data of the problems 

 which are presented to us. In order to exemphfy the size of the fragments with 

 which we have to deal, I have made rough estimates of the diameters of the con-- 

 stituent grains in a series of quartzose rocks. Sometimes there is much variability, 

 but very commonly the majority of the grains are tolerably uniform, both in size 

 and shape. In a slide from the Lickey quartzite, exposed in the railway cutting 

 at Frankley Beeches, grains, often well rolled, ranging from -02" to "03" are very- 

 common. In a specimen of Hartshill quartzite, they range from -01" to -03", but 

 the most common size is a little under '025". In a quartzite from west of Rush- 

 ton (Wrekin) a good many grains range from "03" to -OS". In two specimens of 

 quartzite (white and pale grey) from near Loch Maree, the grains commonly vary 

 a little on either side of -02", while in a specimen of the ' fucoidal quartzite ' 

 (mouth of Glen Logan) much greater variety is exhibited, a good deal of the 

 material being about -01" in diameter, but with many scattered grains up to -03". 

 The grains in a pale grey quartzite, from the Bunter beds at the north side of Can- 

 nock Chase, range from about -01" to -015", and are very uniform. In a liver- 

 coloured quartzite from the same locality they are about as long, but narrower and 

 sharply angular in form. These will serve as examples of what we may call an 

 average, moderately iine grit or sandstone. It is my impression that in a very- 

 large number of ordinary sandstones most of the grains range from about one 

 to three hundredths of an inch. In rocks, however, to which most persons would 

 apply the epithet ' rather coarse-grained,' fragments of a tenth of an inch or more 

 in diameter are common. 



It is extremely difficult to give, in general terms, an estimate of the size of the 

 crystalline constituents of ordinary granites, and the more coarsely crystalline 

 gneisses. But speaking of those which enter into the composition of the ground 

 mass, I should say that the individual quartz grains do not often exceed -03", and 

 are very frequently between this and •02". In the finer-grained granites and more 

 distinctly banded gneisses, and their associated quartziferous schists, about '01" is a 

 common size, while in the finer schists (believed by many geologists to be later in^ 

 date than the aforesaid) they range from -002" downwards, and do not generally 

 exceed •001". Felspar crystals, where they occur, probably do not diifer very 

 materially in area from the quartz, though they are often, as might be expected,, 

 rather longer and narrower ; mica crystals, cut transversely, are often longer and 

 usually much narrower. Of other constituents, as being either rarer or more liable 

 to change, I will not speak in detail. The individual quartz grains, in the compact 

 and glassy varieties of the more acid igneous rocks, are about the same size as 

 those in an ordinarj^ granite. 



Space does not permit me to enter upon the methods of distinguishing between 

 the materials furnished by the diflerent varieties of crystalline schists, gneisses, and" 

 igneous rocks of similar chemical composition. For the most important of these I 

 must refer you to Dr. Sorby's address, but I may add that there are others which) 



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