344 TT. M. Hatchings — Clays, Shales, and Slates. 



Analysis shows a specimen of this slate to contain 496 per cent, 

 of total alkali, a total amount of iron equal to 7-64 per cent, of 

 ferric oxide, and 6'24 per cent, of magnesia. This last is an 

 unusually high figure for a slate of this class. It would correspond 

 to a large amount of chloritic mineral, and the heating shows that 

 a good deal more of such mineral is present than is made evident in 

 ordinary sections ; but still there is not enough to account for so 

 much magnesia, and no doubt much of the latter is combined in the 

 micaceous mineral. 



Although these slates have resulted from very fine-grained 

 deposits, there are still some small flakes of clastic muscovite 

 unmistakably recognizable in the ordinary sections, and still more 

 so in the dehydrated ones; and these seem strikingly to emphasize, 

 by contrast, the above points as to the nature of the main mass of 

 the new micaceous mineral of the rocks. The mere fact that these 

 few small clastic flakes are still so distinct from the main mass, 

 shows how little progress in development has been made, compared 

 with what we see in more regenerated slates, in which great 

 quantities of new muscovite have been formed, and in which 

 original clastic flakes have either been absorbed into the newly- 

 formed materials or are quite unrecognizable among them. In fact, 

 if we compare these ancient and intensely sheared slates with some 

 of the sections of shales previously alluded to, we see that the mica 

 in them is practically the same as that in the more crystalline and 

 felted bands which we have observed in the shales. 



The same applies to strongly-sheared Cambrian slates from 

 Saxony, and to many other examples studied. 



All this emphasizes the fact, frequently pointed out, that intense 

 dynamic action, even allowing for its influence being added to and 

 associated with that of depth-conditions, does not necessarily bring 

 about any commensurate effects, other than purely mechanical ones, 

 in rocks similar in chemical and mineralogical composition, and 

 in nature of origin, to others which we see very highly developed. 



If, now, we turn to the question of the development of rocks in 

 " contact-areas," it will be advisable, for our present purposes, to fix 

 our attention, not so much upon the more intensely altered examples, 

 with a large formation of special contact-minerals, as upon those 

 which are less affected in this way, in which we can observe the 

 increase in the regeneration of the already existing micaceous con- 

 stituent, with the further separation and rearrangement of the 

 chloritic substance. 



In a considerable proportion of cases we have a formation of 

 brown mica in quite early stages of the metamorphism, and this 

 partly complicates the subjects to which we are at present paying 

 attention, as the chloritic substance may be largely, or even wholly, 

 used up in the production of this brown mica. This formation of 

 brown mica also considerably obscures the white mica, and, indeed, 

 there seem to be cases in which the impure complex mineral of the 

 slates is altered, at once and directly, into a pale-brown mica which 

 is something intermediate between muscovite and biotite. 



