W. 31. Hatchings — Clays, Shales, and Slates. 315 



manner described in the paper referred to, it is seen to have taken 

 on quite a reddish tinge, to be more opaque, and quite easily seen 

 even in its minutest fiakelets. 



I have endeavoured to ascertain something as to the chemical com- 

 position of this new micaceous mineral in clays and shales ; but, for 

 obvious reasons, it is a rather unsatisfactory task. I took the two 

 finest portions, called A and B in my former description of these 

 levigations, made one sample of them, and carefully determined in it 

 the percentage of several of the constituent bases. 



In these finest slimes there takes place a concentration, not only of 

 the new micaceous mineral, but also of a large proportion of the 

 rutile-needles, anatase crystals, and ilmenite-flakes of the original 

 material ; also of a certain amount of epidote in small grains and 

 crystals, some very fine quartz-powdex*, and the bulk of the organic 

 matter of the fireclay. One can only make a rough estimate as to 

 the quantity of these things which may be present in the levigated 

 slimes, but, after much examination, I consider that it is safe to say 

 that there is 25 per cent, of material other than the micaceous 

 mineral. 



Making an increase, in this proportion, in the actual figures found 

 by analysis, the micaceous mineral will contain — 



per cent. 



Potash 4'62 



Soda 1-16 



Magnesia 2-94 



Alumina 3231 



A small portion of the alumina will be due to epidote, to which 

 mineral we ma}' also ascribe most of the lime present in the material 

 (0-90 per cent.). 



If we take the figures for the alkalies and magnesia, and approxi- 

 mately for the alumina, and if we consider that with them must also 

 be included a good deal of ferrous oxide, some ferric oxide, and 

 several per cent, of water, we get as near an idea of the constitution 

 of the bases of this complex micaceous mineral as the conditions of 

 the case allow us to obtain; and we see how it is that such a 

 substance, under other conditions, gives rise to a mixture of a mica, 

 more or less developed to muscovite, and of chloritic minerals. 



We get this crystalline development of slates, as before remarked, 

 in very varying degrees, and there are rocks of this class, of which 

 the actual condition is at present universally accepted as being due 

 to " regional " metamorphism only, in which the mica has really, 

 so far as cau be made out, become all muscovite. 1 



If, now, we look at slates in the neighbourhood of intrusive 



1 It may very likely be the case that in some slates, in which a larger 

 proportion of soda than usual is present, the mica formed is largely paragonite. 

 And in other cases there may be a mixture of muscovite and paragonite, or some 

 mica may result which is chemically intermediate between the two. It would rarely 

 be possible to make quite sure of this; but iu any ease the exact nature of this 

 regenerated mica would not affect the considerations involved, and for convenience 

 we may speak of it in general as muscovite. 



