W. M. Hutchings — The Origin of some Slates. 321 



as President of the Geological Society in 1880 (Q.J.G.S. vol. xxxvi.) 

 stated that, though he had at first looked upon this mica as secondary, 

 he was finally of opinion that the main micaceous constituent of the 

 Welsh slates — the mass of small flakes lying parallel to the cleavage 

 — is not secondary, but is simply the compacted material of the 

 original deposit. Dr. Sorby stated that he considered that the mica 

 of granites, etc., "except in special cases, where much iron is 

 present," is not liable to fall to pieces by weathering so as to yield 

 the small flakes seen in slates. He looks upon this small mica 

 as derived from fine-grained micaceous felsites, whereas " coarse- 

 grained granites and f el spathic felsites, even when very micaceous, 

 could yield only kaolinitic clays." 



" Deposits mainly composed of inert substances like quartz, mica, 

 and kaoline, can undergo little further change," though he con- 

 siders that felspar may decompose after deposit, giving rise to quartz 

 and opal. He admits that the crystals of mica which are not 

 parallel to the cleavage, " but lie at all possible azimuths," are of 

 new formation, but says that as regards the rest of it "the whole 

 structure is, in fact, just such as would result from the deposition 

 of material sorted by gentle currents and subsequently compressed, 

 vertically by pressure of overlying strata, or laterallj' by that which 

 gave rise to slaty cleavage." 



Dr. Sorby does not lay any stress on the remarkable diffusion of 

 rutile crystals in so many slates; indeed, in the paper above quoted, 

 he does not do more than indirectly allude to them. At the time he 

 wrote the paper, the nature of the minuter " clay-slate needles " as 

 rutile was only just about being established. 1 Had he taken the 

 rutile into consideration, it is difficult to see how; he would have 

 explained in such cases its presence in simply compacted original 

 mica from felsites, as the mica of these rocks does not usually contain 

 rutile. 



I think the opinion that the mica of granites would not yield a 

 fine-grained deposit is not borne out by the study of the clays and 

 shales of granitic origin ; coarse-grained and fine-grained layers of 



1877, pp. 116-118), though he was very positive as to the non-admissibility of the 

 clastic origin of the rutiles, tourmalines, etc., of the slates, he was much more 

 reserved in any expression of opinion as to the mica. 



Eenard also, who has so deeply studied the slates of the Ardennes, appears to have 

 arrived at views quite opposed to the idea of such rocks being simply compacted 

 original sediments {op. cit.). 



1 The first decided proof that these needles are rutile was given by Cathrein 

 (" Em Beitrag zur Kenntniss der Y\ ? ildschonauer Schiefer und der Thonschiefer- 

 nadelchen," JXeues Jahrbuch fiir Min. Geologie, etc., 1881). 



In dealing with his method of isolation of the rutile, Cathrein mentions the 

 "roundish graius " as well as crystals, and looks on these as "partly imperfectly 

 developed rutile needles." 



Very soon after Cathrein's paper, in the same year, an article appeared in the 

 " Neues Jahrbuch" by Dr. Sauer (" Rutil als mikroskopischer Gemengtheil in der. 

 Gneiss- und Glimmerformation, sowie als Thonschiefernadelchen in der Phyllit- 

 formation"), in which the proof of the nature of the slate-needles is again fully 

 carried out ; and a note by Prof. Eoscubusch states that Sauer had communicated 

 the contents of this article to him. in private correspondence before the appearance 

 of Cathrein's paper. 



DECADE III. — VOL.' VII. — NO. VII. 21 



