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Published online by Cambridge University Press: 01 May 2009
These notes are in continuation of my former paper “On the Probable Origin of some Slates” (Geological Magazine for June and July, 1890). Since its publication I have endeavoured to obtain a little further knowledge of the constitution of the fireclays of the Coal-measures, which I regard as so well calculated to throw light on the origin of many slates, and especially I desired to get, if possible, a little more evidence as to the supposed regeneration of mica in them, in connexion with the intimately associated question of the rutile-needles.
page 167 note 1 In those cases where the separated rutile is arranged in approximately hexagonal star-forms, more or less conformable to the symmetry of the containing mica, I consider that it is due to other causes than weathering, viz. sometimes to original crystallization, but more often to powerful dynamic action, and that it is in the same category as the symmetrically arranged rutile in some micaceous ilmenite, and in sagenite.
page 168 note 1 It is stated in works on geology that the “underclays” of the Coal-fields are unstratified, and though I am not aware that, this is so uniformly the case as to allow of its being given as a universal rule, still it is no doubt true enough when taken as referring to the clays occurring with workable seams of Coal. It is not by any means so as regards beds of exactly the same materials which occur in the same Coal-fields, either without any immediate connexion with Coal-seams, or only with very thin seams, far too thin to work. This unstratified condition of many fireclays is probably due simply to the crumpling and crushing they have undergone in connexion with the compression of the materials out of which the Coal was formed. It is also, no doubt, quite true as stated in the book (first suggested by Sterry Hunt), that Clays on which the Coal-vegetation actually grew were deprived of much alkali by the plants. It is therefore more desirable in studying these deposits, as bearing on the origin of slates, to select beds which have not had the stratification disturbed and which have no Coal in actual contact. This is the case with the beds at Seaton Cliffs, which I have used so much as my principal type and source of material.