^{page 314 note 1} There is no harm in here reiterating a caution as to the necessity of very thin sections, this being especially important for those cut from dehydrated specimens. Personally, I have never seen any made by English cutters which are of the least value for studying this class of rocks. It is decidedly a case of things “made in Germany” being better! The vagueness and indistinctness of many descriptions given of slates, etc., are well accounted for when one sees the sort of paving-slabs which have often done duty as “thin sections,” and when one sees them mounted on such thick glasses, and covered with such thick covers, that no use of higher powers or proper illumination is possible.

^{page 315 note 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 in any case 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.