350 W. M. Hutchings— Clays, Shales, and Slates. 



chloritic matter, and the sheaves and radiating groups are partially 

 composed of it. 



Apart from mere size of constituents, it would not be possible 

 to find more characteristic recrystallization than we see in this 

 and some other of these shales, the whole mode of development 

 being the same as at granite-contacts. 



In connection with the subject of certain developments in 

 sedimentary rocks, apart from any demonstrable contact-action, 

 and with which developments dynamical and regional causes are 

 credited, I would like again to allude to some of the occurrences 

 in the Ardennes. In a former paper (Geol. Mag.. March and April, 

 1895), I pointed out the occurrence of abundant "spots" in the 

 coticule of Viel Salm, and in the dark slate with which it is 

 associated. I particularly described these spots from their special 

 points of interest, as they are perfectly colourless and absolutely 

 indistinguishable in ordinary light, and are uniformly dark and 

 isotropic between crossed nicols. 



The thin bed of pale-yellow coticule does not differ from the 

 accompanying dark rock in the nature or degree of its development. 

 But it does differ very much in the fact that it is free irom iron 

 ores or other dark matter, so that sections of it enable us to study, 

 without impediment, its mineralogical and structural condition. 

 "We see that its mica is all quite clear muscovite, and that nothing 

 is left of any original impure micaceous mineral. No chloritic 

 matter can be made out in sections from the specimens I have 

 examined ; what there is of it is quite colourless and invisible in 

 thin slices. The rock is unusually free from iron. Eenard gives 

 in its analysis only 1-05 per cent, of ferric oxide and 0-71 per cent. 

 of ferrous oxide, with 1-13 per cent, of magnesia. He calculates 

 from these data that the rock contains 8 - 74 per cent, of " chlorite " ; 

 but, of course, this is only an approximation, as the calculation 

 supposes a definite composition of the chloritic mineral, which 

 there is not any reason for thinking that it possesses. 



A thin transverse section cut from a specimen of coticule after 

 dehydration, brings to view the chloritic mineral as a light greyish- 

 brown substance now quite conspicuous among the mica at some 

 parts of the slide ; it is not evenly diffused all over it. The 

 muscovite is not in any way altered by the heating. 



A very interesting point brought about by examining the 

 dehydrated rock, is that the spots in it are not in any way affected, 

 but remain just as indistinguishable in ordinary light as before. 

 Whatever is their real nature, it is clear that they do not consist of 

 a hydrated chloritic material. And, indeed, it must be borne in 

 mind that, though spots of the chloritic nature we have been 

 considering are so frequent as to make out a very large proportion 

 of these interesting bodies, there are also other varieties of them 

 which do not come under this description, and which are due to the 

 formation of other combinations during the recrystallization of the 

 rock-constituents. 



