W. M. Hut clung i— Rocks of Great Whin Sill 75 



aggregates giving more or less black-cross figures in polarized liglit. 

 Intimately mixed and interwoven with this new mica is an 

 abundant cbloritic minei'al. This chlorite forms part of the sheaves 

 and spherulitic groups, and all over the slides is seen to have been 

 formed flake for flake with the mica, as a result of one and the same 

 process. No such chlorite exists in the unaltered beds ; as I have 

 previously pointed out, both it and the white mica are the result 

 of a splitting-up and higher development of the impure and 

 complex micaceous mineral of the clays and shales. In some cases 

 there is a certain amount of biotite formed, with a similar mode of 

 occurrence, but this is not frequent among these rocks. The 

 formation of " spots " may also be seen o.n a copious scale in some 

 specimens ; and these spots, though small, are exactly analogous to 

 the larger ones seen at somei granite contacts, being due to the 

 aggregation of the chloritic material which is separated out during 

 the recrystallization of the rock constituents. 



One or two special examples of the alteration of these pure 

 shales may be given in illustration. Thus, a specimen from 

 Kowntree Beck, taken 18 feet below the Whin, is very highly 

 altered but still contains good fossils. An analysis of it gives — 



Silica 

 Alumina ... 



Ferric Oxide 1 



Lime 



Magnesia ... 



Potash. 



Soda 



Water 



100-78 



This composition shows the rock to have been originally of the 

 nature of a fireclay, closely resembling some of the series of which 

 I published analyses in a former paper (Geol. Mag., 1894, Dec. IV'^, 

 Vol. I, pp. 36-45 and 64-75). It is now a muscovite-chlorite rock, 

 with abundant " spots " all over it. Into these spots is concentrated 

 nearly all tlie pigmental matter of the rock, together with chlorite 

 and numerous dark grains and microlites, so that the spots are dark 

 in a light field. Parts of this field are almost clear and colourless. 

 In polarized light they are seen to consist of an interlacing mass 

 of muscovite and pale chlorite. Sheaves and spherulitic bundles of 

 mica and chlorite abound all over the section, and there is no sigu 

 of any definite orientation of these minerals in any direction. The 

 entire rock is crowded with small grains and cr^'stals of ratile re- 

 crystallized from the original " needles " of the shale. 



Another interesting spot-rock comes from near High Force. In 

 ordinary light a sectioQ of it shows a sort of marking off into 

 roughly polygonal, or approximately circular, clear spots, framed iu 



' In this and following analyses all the iron is reported as ferric oxide, no special 

 determination having been made of the portion which is always present as ferrous 

 oxide. This often causes more or less excess in the totals, but is not of any im- 

 portance for the purposes for which these analyses were made. 



6-99 



