W. M. Hutchings — Further Notes on Fireclays, etc. 167 



appear, — the latter scarce, — all completely rounded and many carrying 

 more or less a skin of rutiliferous " paste," in spite of all the 

 washing and agitation. I think it is really corroded on to the sub- 

 stance of the grains in many cases. A few beautiful little crystals 

 of anatase, both pj^ramidal and tabular, are not missing. 



Looking at the facts stated, the one which appears to have most 

 bearing on the main point concerned is the pretty strict upward 

 limit of size of the rutiliferous flakes. 



If we suppose the rutile-needles to have been contained, as such, 

 in the original clastic material as it was deposited in these beds, 

 then we should find no such limit of size. If the rntile were 

 contained in the micas, for instance, we could not well find any 

 reasoning to explain why only flakes up to a certain small size 

 should be rich in it, while those of larger size, from which the 

 smaller ones must have been derived (on the supposition that what 

 we now see is all primary deposit), should be absolutely free from it. 

 These rutile-needles are, as I have before maintained, the key to 

 the whole question. Some writers have simply ignored them, others 

 make very light of them. Thus Roth (Chemische Geologie, vol. ii. 

 1887, p. 586) doubts that any proof has been given of the authigenio 

 origin of the rutile-needles in slates ; and again in his latest volume 

 (Allgemeine Geologie, vol. iii. 1890, pp. 169-170) returns to the 

 subject and contests the views of Ilosenbusch and Lossen as to the 

 secondary origin of the colourless mica of clay-slates, dealing with 

 the rutile-needles by saying that " separation of the titanic acid of 

 weathered micas as rutile is often enough observed." 



As regards this supposition, that the rutiles in question in these 

 fireclays may be due to any simple weathering of biotite, the same 

 argument as to the size of flakes appears again to be difficult to 

 answer. We might, perhaps, saj that the larger the flakes the less 

 they would be susceptible to such weathering. (This is not the 

 case with the process of decay of biotite, pure and simple, as 

 described in my former paper. The large flakes suffer as much 

 as smaller ones. Nor does it seem true of biotite in any sort of 

 weathering; chloritization, for instance. Big flakes are attacked as 

 easily as little ones, the extreme fissility apparently counterbalancing 

 the greater size.) 



But even on this supposition no approach to a sharp line would 

 be seen ; the diminution of the amount of weathering and of the 

 resultant rutile would be more or less gradual in proportion to size. 

 Of course we know that biotite does sometimes give rise to rutile 

 directly during weathering. What are the conditions, either in the 

 original biotite or in the method of weathering, which cause this, we 

 do not know ; but we do know that it is relatively rare compared 

 with the universal weathering of biotite without any formation of 

 rutile.^ 



^ In those cases where the separated rutile is arranged in approximately hexa- 

 gonal 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. 



