320 W. M. Hutclrings — The Origin of &ome Slates. 



the biotite and quartz of contact-slates, the abundant rutile-needles 

 of the original slates (e.g. Sbap, Skiddaw). 



It may be noted that the chlorite which is so plentiful in some 

 Welsh slates, and in most of the Cornish roofing-slates and allied 

 material, appears to be wholly of later date than the rutiliferous 

 sericitic mica, and in many cases to be due to subsequent deposit 

 from infiltrations, such chlorite never showing any inclosed rutile, 

 ilmenite, etc. 



In other cases, however, it appears to be due to the more advanced 

 stages of dynamic action, being then intergrown or twinned with 

 muscovite, vertical or more or less highly inclined to the cleavage 

 of the slate. In this manner of occurrence it sometimes contains 

 rutile, and notably so in some Tintagel rocks. 



The white mica which is formed with or without chlorite (or with 

 ottrelite at Tintagel) lying otherwise than with the cleavage, appears 

 to be true muscovite. It is present in some Welsh slates, absent 

 from others, and very abundant in nearly all the Cornish slates, 

 being always rich in inclosed rutile. It can all be seen to be due 

 to later stages of dynamic action than that which arranged the main 

 mass of the mica in one plane. 



It may be well to bear in mind that though opinion is now mainly 

 in favour of the belief that the mica of such slates as we have been 

 considering is mainly or wholly secondary, and produced in situ, there 

 is one very great authority on the subject who arrived at a quite 

 opposite conclusion after much research. 1 Dr. Sorby, in his address 



1 At about the same date as Dr. Sorby's address, Pfaff published a paper on some 

 more recent clay-slates (" Petrographische Untersuchungen iiber die eocanen 

 Thonschiefer der Glarner Alpen," . Sitzungsbericht der K. Bayer. Akad. der 

 "Wissenschaften, 1880). The examination of these slates and shales showed a 

 "surprising amount of resemblance to the Palaeozoic clay-slates" down to the 

 presence of the " clay-slate needles." Pfaff arrives at the same conclusion as does 

 Sorby as to the slates being simply compacted original sediments ; but while Sorby 

 seems to have formed this opinion from the study of the slates simply, and from 

 some reasoning as to what might be looked for as a result of disintegration of granites 

 and other rocks, Pfaff bases his verdict more on the fact that he has studied clays of 

 various ages (including Carboniferous, Triassic, Jurassic, and Tertiary), in all of 

 which he sees the microlites (rutile) and the micaceous base, as in the slates. 



If the result of a full study of clays and shales were to convince the observer that 

 the rutile, etc. , and the micaceous base are all original deposit, then the very great 

 similarity in these respects of clays and slates would go far to compel the adoption 

 of the same conclusions as are advocated by Sorby and Pfaff. 



As shown above, the key lies in the selection for study of clays whose exact and 

 direct origin can he pretty clearly demonstrated. In such clays I do not consider it 

 is possible, in view of all the characteristics, to look on the rutile, tourmaline, mica, 

 etc., as of clastic origin. 



Pfaff' s paper is abstracted and criticized by Eosenbusch (Neues Jahrbuch fur 

 Min. etc., 1881), who expresses very decidedly contrary opinions to those of Pfaff, 

 stating that he holds the cardinal point as to the question of the formation of slates 

 to lie in the large amount of mica, from which he concludes that chemical processes 

 have played a large part. " Gewiss wurde das Material zu den Thouschiefern 

 mechanisch herbeigefuhrt, der Mineralbestand aber der vorwiegend glimmerhaltigen 

 und felspathfreien Abtheilung derselben ist gewiss durch metamorphe Processe 

 bedingt." 



In dealing with the same c|uestion on a previous occasion (Die Steiger Schiefer, 



