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been a deposit of micaceous mud, for the rounded grains 
have every character of being water-worn ; and in the lime- 
stone of Rhiwlas, near Bala, which consists almost entirely 
of such grains and flakes of mica, and fragments of encri- 
nites, their organic structure is as perfect (or even more so) 
as in any limestone with which I am acquainted, though I 
have prepared and examined thin sections of several hundred 
specimens of every geological period ; and so much so, that 
any material amount of metamorphism is wholly out of 
question. When deposits of decomposed felspar have been 
acted on by great heat, they are, as it were, baked into a 
natural porcelain, but no such grains of mica are formed. 
Usually, besides mica, there is found in good roofing slate, 
like that at Penrhyn, a certain proportion of decomposed 
felspar, a few minute grains of quartz sand, and granules 
of phosphate of iron. The red tint is produced by the 
presence of very numerous minute crystals of peroxide of 
iron, and the dark by those of pyrites. From such slate 
there is every gradation to those containing little or no mica, 
but made up of more or less fine quartz sand, and decom- 
posed felspar, in very variable proportions ; but these have 
only an imperfect cleavage. Other slates, as is well known, 
contain much chlorite and other minerals. On the present 
occasion I shall chiefly confine myself to the consideration 
of such slate as has a perfect cleavage. 
If a thin section of a rock not having cleavage be ex- 
amined, which has a similar mineral composition to those 
which, when having it, form good slates, it will be seen that 
the arrangement of the particles is very different. For 
instance, the well known Water-of-Ayr stone has no clea- 
vage, but shows more or less of bedding. It consists of 
mica and a very few grains of quartz sand, imbedded in a 
large proportion of decomposed felspar ; the peroxide of 
iron being collected to certain centres, and having the char- 
