76 PEOCEEDIITGS OP THE GEOLOGICAL SOCIETY. 



Cleavage, in the Green Slates. 



In studying the green slates, to try and learn the nature of the 

 original material, it is very desirable to examine specimens which 

 have little or no cleavage. When the rock has been greatly com- 

 pressed, and a perfect cleavage developed, the structure of the 

 constituent fragments has been so much changed that it is almost 

 impossible to say what they originally were. Thus, for example, 

 the brecciated slates of Eosthwaite suffice to establish beyond all 

 doubt the mechanical origin of the cleavage ; but the fragments are 

 so compressed in the plane of cleavage that it is impossible to be 

 sure whether the greater part were f elsite or an older slate. There 

 does, however, appear to be sufficient evidence to prove that they 

 contain not only fragments derived directly from volcanic ash, but 

 also others which, if originally of similar nature, must have been 

 stratified, consolidated, altered, and broken up again, perhaps 

 during volcanic outbursts, and not necessarily portions of rocks of 

 much higher antiquity than the main mass of the slates. 



Slates in Different districts. 



Having now, as I think, proved that we must not seek for true 

 representatives of some of our slate rocks amongst more modern 

 British deposits, it will be well to consider the general character of 

 some of our older rocks. Judging from what I have been able to 

 ascertain from those districts which I have more completely ex- 

 amined, little reliance ought to be placed on conclusions drawn 

 from a very limited number of specimens ; for considerable varia- 

 tion may depend on local sorting; and what I am now able to 

 state respecting some other districts must be looked upon as 

 provisional, and possibly only partially true. I have had consi- 

 derable difficulty in reconciling my own observations with those of 

 some who have treated on this subject. Possibly this may be due 

 to actual differences in the slates of different districts, but may 

 also be due to a difference in the methods of observation, leading 

 us to a difference of opinion respecting the true shape and nature 

 of the constituent particles. Some of my conclusions may be 

 wrong, since occasionally it is extremely difficult to ascertain 

 what is the truth. I now give solely the results derived from my 

 own examination of specimens collected and prepared by myself. 



I may here point out that the general optical character of a fine- 

 grained slate consisting mainly of mica, differs very greatly from 

 that of a more normal deposit. When a section cut at right 

 angles to the cleavage is rotated in polarized light, it becomes, over 

 nearly the whole surface, very bright and much darker at different 

 azimuths, like a doubly refracting crystal, whereas there is little 

 or no such change in the case of true clay slates of the normal 

 granular type, containing much kaolin and very little mica. 



The only specimen of slate from the Loch- A we district that I 

 have examined is of perfectly normal type, but is so much altered 

 that it is really a fine-grained schist. 



