W. M. Hutchings — The Origin of some Slates. 319 



In no slate from Tintagel district have I detected a single grain 

 of original quartz, nor any felspar. Secondary quartz is not 

 uncommon, and all cracks are filled with veins of it. Even where 

 no sign of it is seen with the microscope, analysis shows that moi*e 

 silica is present, than could be accounted for in the mica, etc. Thus 

 one such sample from Tintagel quarry contained 50 3 per cent, of 

 silica, showing that an excess of that substance is diffused in some 

 form among the other components. 



These Cornish slates appear to have undergone far more dynamic 

 metamorphism than the Welsh, and we should expect the obliteration 

 of original grains of the deposit to have been more complete. 

 From the almost total absence of epidote from the typical slates of 

 Tintagel and Delabole, the smallness of the zircons (the one remnant 

 of original deposit), and the large amount of rutile, I would infer 

 that they were laid down as a very fine silt indeed, of granitic origin 

 mainly. 



In all these slates, Welsh or Cornish, however, the proportion 

 and the size of the other constituents may vary, the fundamental 

 base of mica is much the same in nature so far as can be made out 

 with the microscope alone. I am unable to ascertain that any of 

 the roofing-slates contain any residue which could be identified with 

 the granular, impure "kaolinitic" material of the clays and shales. 

 All this is apparently altered, the processes seen in progress in 

 those earlier stages of metamorphism having been completed under 

 the much longer and more intense dynamic action which the 

 slates have undergone, clastic muscovite having also nearly totally 

 disappeared. 



This action has resulted, among other things, in dehydration, the 

 combined water in slates being very much less than in shales and 

 clays, however micaceous these may be. This combined water 

 ranges about 4 to 5 per cent, in slates. It is often double and more 

 in fireclays and shales. 



So far as I am aware, we are not in possession of any exact 

 knowledge as to the chemical constitution of the mica forming the 

 base of the slates, and it would not be easy to obtain, owing to the 

 difficulty of isolating any of this mica. We have an immense mass 

 of analyses of " clay-slates " in bulk, and magnesia figures in nearly 

 all of them. Thus, in 80 analyses discussed by Bischof (Chemische 

 und physikalische Geologie), it ranges from a trace to 11*71 per 

 cent. This maximum is quite exceptional, a good average would be 

 from about 0-7 to between 2 and 3 per cent. Many slates contain 

 chlorite, which may account for the magnesia in some cases, but 

 many do not contain that mineral, or only very slightly. That 

 magnesia is present — that the materials for biotite are contained in 

 the slates — is shown also by the plentiful formation of that mineral, 

 which is such a usual accompaniment of contact metamorphism, and 

 which also often takes place in advanced dynamic metamorphism. 

 I have not seen this dynamo-metamorphic biotite in any of my 

 Welsh specimens, but it is strongly developed in some of the 

 Cornish, and slightly in many. It is interesting to see, in among 



