W. M. Hatchings — Clays, Shales, and Slates. 343 



of phosphoric acid, corresponding to 6540 per cent, of phosphate of 

 lime. The proportion of iron and alumina is only 1-51 per cent. 

 Here, then, we have in the London Basin the same kind of rich 

 phosphate which is worked in Picardy. 1 



IV. — Clays, Shales, and Slates. 



By W. Maynard Hutchings, F.G.S. 



(Concluded from the July Number, p. 317.) 



I HAVE recently been examining a series of specimens of 

 Silurian slates from Scotland, specially collected because of 

 the very intense shearing they have undergone. These and the 

 accompanying other rocks are all crushed and torn out in 

 a most remarkable degree, and the microscope shows how severely 

 their constituents have been acted upon mechanically. Yet the 

 crystalline and mineralogical development is seen to be but very 

 moderate indeed. The micaceous mineral is all in one plane, 

 and transverse sections, in polarized light, show that it is a felted, 

 wavy, and puckered mass of interwoven flakelets, with chloritic 

 substance intimately diffused among it, and still more apparent 

 in separate little streaks and lenticles. The small amount of 

 quartz present is rolled out into long, thin lenticles also. But the 

 micaceous mineral itself is not at all far advanced, either as to size 

 of its flakes or the nature of the mineral. It is still an impure, 

 rather deeply-coloured greenish-yellow substance ; not any of it has 

 arrived at the stage of muscovite, and dehydration shows, in a 

 remarkable manner, how very considerable its impurity still is. 

 In its original condition, the slate gives sections which present 

 a very uniform appearance under the microscope. The colour is 

 the same all over, and the only indication of the presence of bands 

 of varying nature is found in a very slight increase, in some layers, 

 of the coarseness of grain and of the number of quartz-lenticles, 

 and in the diffusion of the chloritic substance. But after heating 

 the specimen, the effects seen in the sections are most striking. 

 There are now many bands of different colours, yellow, red, greenish- 

 brown, and brown, presenting very strong contrasts, and showing 

 that noticeable differences in chemical composition must have existed 

 in different thin layers of the seemingly homogeneous slate. Not 

 only the interwoven chlorite, but also the micaceous mineral, has 

 taken on these tints in very decided degrees. It must have been 

 unusually impure. It has lost, after heating, nearly all its action in 

 polarized light. 



The lenticles and the small grains of quartz have also become 

 much more apparent than before heating, and, indeed, this case 

 serves as a good example of how useful this method can be made 

 in assisting us to obtain a knowledge of the nature of rocks of 

 this class. 



1 A specimen of the "rich phosphate" of Taplow was exhibited at the 

 Geologists' Association at the ineetiug on July 3, 1896, and is now placed in 

 the Museum of Practical Geology. 



