168 W. M. Hutchings — Further Notes on Fireclays, etc. 



But in these deposits, derived from granite-waste, the biotite is 

 clearly seen not to give rise direct to any crystals of rutile, any 

 more than it does in most granites, etc., where it is undergoing 

 decay in situ. 



The effect on my own mind of studying the slates, passing from 

 these to the fireclays and associated shales (for reasons formerly 

 stated), and back again to the slates, having approached the matter 

 without any bias one way or other, is that I cannot come to 

 any other conclusion than that the main mass of mica, — all the 

 rutiliferous mica,- — of the slates is a new formation, posterior to 

 sedimentation ; that in the fireclays and shales the same thing has 

 commenced and progressed in the same manner up to a certain 

 stage ; and that most of our slates were produced from deposits of 

 closely similar nature by a continuation and intensification of the 

 conditions involved. 



I must thank Dr. Irving for his very kindly and encouraging 

 remarks (in the Geological Magazine for December) concerning 

 the value of my little study of the clays, etc., but I do not think 

 I can quite follow him in the conclusions he draws. 



For one thing, will it be quite safe to say that the fireclays of 

 Northumberland (or any other Carboniferous clays and shales) have 

 undergone no dynamic metamorphism whatever, even though what 

 has taken place did not suffice to alter the deposits into hard cleaved 

 slates ? 



It has suggested itself to me that possibly the more or less close 

 association of these beds with the yielding coal-seams may have largely 

 tended to preserve them from any of the severer effects of pressure, 

 etc. ; ^ but I am quite inclined to think that we are justified, on 

 geological evidence, in assuming that they have taken pai't in con- 

 ditions which could well be classed as " dynamic." 



If I have, as Dr. Irving says I have, " simply ripped up " the 

 idea of the "dynamometamorphic" origin of the secondary minerals 

 in slates, I can only say that I had it not in my mind to do this 

 savage and gory deed at all. If I have done it (which I do not see), 

 then I am in the painful position of having unintentionally ripped 

 up a friend, for I certainly believe that the completion of the 

 regeneration of the materials of the deposits resembling the fireclays, 



^ It is stated in works on geology that the " underclays " of the Coal-fields are 

 unstratijied, 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-iields, 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 con- 

 nexion 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. 



