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



The analysis A shows an unusualby low amount of iron, but with 

 this exception the composition corresponds pretty closely with 

 similar rather quartzy shales from other coalfields, as, for instance, 

 with No. 1 of a series of six analyses which I published (Gkol. 

 Mag., January, 1894, pp. 36 and 64) of fireclays from a borehole 

 at Aspatria, near Carlisle. Notably, the relative proportions of 

 potash and soda, the great excess of the former, are here maintained. 



The anatysis B shows in general a similar composition, but this 

 shale contains a noticeable amount of grains of a carbonate (probably 

 of lime, magnesia, and iron) more than I have seen in any other 

 Coal-measure clay or shale. Also, the excess of potash over soda is 

 less than is usual in these deposits. I am not aware from what 

 exact position in the Carboniferous beds the specimen was taken. 



The Welsh shale is a decidedly quartzy and coarse-grained speci- 

 men, as compared with the finest-grained, hard fireclays from the 

 borehole at Aspatria and elsewhere. It still shows a considerable 

 amount of clastic muscovite in good-sized flakes, and here and 

 there even a bit of not yet wholly decayed biotite may be seen. 

 These Welsh specimens are very deeply coloured with organic 

 pigment, which makes sections of them very opaque and bad to 

 examine, in comparison with the much less pigmented grey shales 

 and clays from some other sources ; but the general nature is still 

 easily made out, being the same as in the other cases. There are 

 the rutile-needles, small anatase crystals, and other minerals as usual, 

 and the finest-grained "base" of newly-formed micaceous material. 



The same remarks apply in all principal respects to the Isle of 

 Man shale, though it is not so quartzy as the Welsh specimens, and 

 contains, as before stated, some carbonates disseminated through it. 



So far as concerns the physical condition, and development towards 

 slates, these specimens differ only from those previously examined 

 inasmuch as they are considerably less advanced than many which 

 have come from very much smaller depths. They are, in fact, just 

 very hard clays; they can easily be cut with a knife, and when 

 ground with water they are quite plastic. They have, in fact, not 

 made any advance at all beyond that of the considerable chemical 

 changes and the very moderate physical progress which is common 

 to all the clays and shales of the normal Carboniferous beds ; and 

 it does not appear that we can look for any gradually higher stages, 

 among these deposits, to assist us to bridge over the interval between 

 6oft shales and well-developed slates, or to understand what it is 

 which takes place in them during the passage through the inter- 

 mediate stages. 



Such fully-developed slates we can only study, in this country at 

 least, among the older formations, with the exception, possibly, 

 of certain local occm-rences in the Culm of North Cornwall, etc. 

 In other regions highly -developed slates are found among rocks 

 younger than the Carboniferous. It is not, by any means, simply a 

 question of geological age. 



In considering these higher phases of " slates," as contrasted with 

 soft shales and indurated clays, it is not so much the mechanical 



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