266 W. M. Hutching* — The Origin of some Slates. 



undergone some considerable disturbance, having been thrown info 

 folds, while at several points the clay beds are faulted and puckered. 



Some of the coarser, harder, more micaceous deposit was taken out 

 and subjected to detailed examination. Sections were prepared of 

 it, and its component minerals wei'e also separated, as far as prac- 

 ticable with materials of this nature, by levigation and by use of 

 Sonstadt's solution. 



It is of the greatest importance to always, whenever possible, 

 study clays, soft shales, etc., mounted as continuous sections as well 

 as in crushed form in water and balsam. It is only by so doing 

 that the relationship of the various constituents to each other can be 

 properly observed. Even with quite soft clays it is practicable to 

 prepare sections; or perhaps the word "section" hardly applies in 

 such cases, and one might call it a continuous layer or film. 



The material, well dried but of course never so heated as to lose 

 any of its combined water, may be cut into pieces of suitable size 

 with a knife, levelled off on a hard rough surface (good emery-cloth 

 does excellently), brushed clear of dust and smoothed, without water, 

 on a ground-glass plate ; then warmed again and cemented in the 

 usual way to a slide. The piece, which must be thick for the fore- 

 going operations, is now cut down with a knife, rubbed down pretty 

 thin on emery-cloth and finished by very careful work, with water 

 only, on a glass plate. 



Exceedingly thin films may be got in this way, but the outer 

 edges, or the thinnest margins of holes that may have been worn 

 here and there, will be found of most value for the examination of 

 fine-grained clays under high powers. 



Sections of the material in question show that it is made up 

 mainly of mica, both muscovite and biotite, with grains of quartz 

 and felspar, and that zircons and other accessory minerals are present. 



A quantity was crushed to powder and suspended in a solution 

 of 3-1 sp.g. The minerals which fell out were zircons, numerous 

 and of rather large size, garnets in angular colourless fragments, 

 crystals and fragments of rutile of the usual sort found in sands. 

 Anatase was seen as one or two small pyramidal crystals of bluish 

 colour, and also several beautifully perfect thin tabular crystals, pale 

 yellow to colourless. The largest observed was -3x0 inch in diameter, 

 others being ^ott and less. They are perfectly sharp and unworn 

 in any way, and are doubtless formed in situ in the deposit, as has 

 been demonstrated of similar crystals by Thurach. (" Ueber das 

 Vorkommen mikroskopischer Zircone und Titan-Mineralien in den 

 Gesteinen," Wiirzburg, 1884). There is also a good deal of tour- 

 maline in crystals and angular fragments, and one or two grains of 

 sphene. Further, numerous colourless, transparent, tabular crystals 

 of rhombic shape were observed. The long diameters of the lai - gest 

 crystals measm-ed were -^0 inch, but many are much smaller. The 

 acute angle of the rhombs is 78°. An obtuse bisectrix emerges on 

 the face of the tablets and the bi-refraction is positive. These 

 crystals thus correspond in all respects with one of the forms of 

 baryte (I think known as the "chisel-shaped" crystal), and taken 



