142 Henry Clifton Sorby on the 



probably, the true average absolute condensation ill such rocks, has 

 been to about one-half of the original volume. This must have 

 resulted chiefly from the forcing of the particles more closely to- 

 gether, so as to fill up the spaces left between them, when only 

 touching each other ; and their very close packing, as seen in thin 

 sections, agrees well with this supposition. 



These amounts of change of dimensions vary considerably in dif- 

 ferent cases, but they agree most perfectly with that indicated by 

 the contortions of the beds in their immediate vicinity, and also 

 most closely correspond with that deduced from the breaking up of 

 the rounded grains of mica. 



The power most generally useful in examining slate rocks, is 

 about 400 linear ; but higher and lower are of course valuable for 

 particular purposes. It is almost indispensable to use a polarizing 

 microscope, and there should be such contrivances as to give a good, 

 bright, polarized light with high powers. The physical structure 

 and optical properties of the minerals found in them, are such that 

 they can be identified with great certainty, even when in grains less 

 than T q\ Q-th of an inch in diameter. 



Some slate rocks, as for instance the pencil slate of Shap, consist 

 almost entirely of rounded grains and minute flakes and granules of 

 mica, varying from about T ^th to IF ^ ^ ^-th of an inch in diameter, 

 but chiefly under T - \ ^th. I do not believe that this is in the least 

 due to metamorphism, but has been a deposit of micaceous mud, 

 for the rounded grains have every character of being water- worn ; 

 and in the limestone of Rhiwlas near Bala, which consists almost 

 entirely of such grains and flakes of mica, and fragments of en- 

 crinites, their organic structure is as perfect, or even more so, than 

 in any limestone with which I am acquainted, though I have pre- 

 pared 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 crys- 

 tals 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 decomposed felspar, 

 in very variable proportion ; 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 con- 

 fine myself to the consideration of such slate as has a perfect cleavage. 



If a thin section of a rock not having cleavage be examined, which 

 has a similar mineral composition to those which, when having it, 



