76 W. M. Sidchings — Rocks of Great Whin Sill. 



darker greyish and brownish pigmental matter. The spots are 

 mostly of a very pale greenish colour, and proper illumination enables 

 us to see countless iJakes and crystals of a chloritic mineral. With 

 crossed nicols the whole slide is resolved into a network of 

 muscovite flakes, lying again in every possible direction, and amid 

 the brightly polarizing mass of this mica the chloritic spots are 

 more or less dark and isotropic. In some the transition from the 

 bright frame of mica is quite sharp ; in others mica projects more 

 or less into the spot, and only the centre is free. Many spots show 

 a field of quite isotropic, pale-green substance, in among which a dim 

 and speckly fine-grained mosaic polarization is discernible. These 

 spots are again in all respects exact counterparts of what may be 

 seen at some granite-contacts. It is carious to observe that, 

 whereas in the previous example the dark pigmental matter has 

 concentrated inside the chloritic spots, in the present case it has 

 remained completely outside them, and is mixed in with the mica. 

 I have made no analysis of this rock, but it shows only a very 

 small amount of recrystallized quartz, and is no doubt very closely 

 the same in composition as the last. No trace of clastic muscovite 

 remains in either of these, and, indeed, in nearly all the highly 

 altered fine-grained shales examined, it has absolutely disappeared 

 and entered into the same complete recrystallization which has 

 affected the main mass of secondary micaceous and other material 

 of which the shales and clays were composed. 



Perhaps among these very fine shales examined, the most interesting 

 case is shown in a specimen from near Winch's Bridge, in Teesdale. 

 It is from a body of rock which has been caught up by the Whin. 

 It contains — 



Potasli 5-71 per cent. ) 7 on 



Soda 1-49 „ 1 ' ^" 



Water 7-40 „ 



This is very nearl3r the maximum of alkali which I have found 

 in any of the carboniferous shales and clays. It can have contained 

 but little quartz. When it is examined under the microscope it 

 is seen to consist, to a very large extent, of the peculiar substance 

 which I have> previously described in detail as being found in varying 

 quantity in so many altered rocks around granites (Geol. Mag., 

 January and February, 1891). I traced and described its various modi- 

 fications and developments, and endeavoured to show the probable 

 nature of its origin and the part it plays in the changes going on 

 during contact-metamorphism. There seems every reason to regard it 

 as a product of the solution, or "aqueous fusion," of original materials 

 preliminai-y to recrystallization. Sometimes we see it in a quite 

 amorphous state. Its first stage of development from this shows 

 a faint minutely-speckly polarization. At very thin edges, with high 

 powers and suitable illumination, it is seen to be very finely granular. 

 We can see it in contact-slates in all stages of evolution, from 

 the first appearance in it of very few and small mica-flakes, up to 

 a full development of new mica out of it. 



