124 W. M. Hutchings — An Interesting Contact-Roch. 



distinguished from the original clastic grains of the shale; not one 

 could by any possibility be mistaken for original material, even 

 if only for the reason that they are full of minute rutiles and other 

 material from whicli the clastic grains are quite free. 



Here and there white mica is developed in larger flakes, lying 

 in all directions, and, as before remarked, there is a good deal of 

 the mineral in minute flakelets. But, on the whole, mica is a 

 subordinate mineral in these special slides. 



The clastic quartz-grains are a very interesting study. They are 

 all more or less corroded and altered round their outer portions. 

 The angular outlines of the original grains are in nearly all cases 

 quite preserved, but the outer portions are no longer quartz. They 

 are seen to be attacked and altered by the material of the isotropic 

 base, and the altered rim so produced contains a large amount of 

 white mica. In many grains this mica is seen to penetrate well 

 into the quartz in little bays and gulfs, or as isolated streaks. It 

 is largely in the form of sheaves and radiating groups of flakes. 



In some cases the corrosion and alteration of the quartz-grain has 

 taken an even more striking form. Thus, e g., one grain with an 

 average diameter of -4V inch is altered to about half its extent. The 

 inner portion is still pure unaltered quartz, with its cavities and 

 bubbles ; the outer part is faint yellowish in colour, and quite 

 sharply marked off in ordinary light from the original mineral. 

 With lowered condenser it shows a granular and fibrous structure ; 

 in polarized light it is seen to consist of clear transparent isotropic 

 matter, and to contain numerous sheaves and spherulitic bundles of 

 polarizing fibres and needles, which can be optically identified as 

 felspar. The outer boundaries of the original grain are quite sharply 

 marked off against the surrounding mass of hard mosaic and iso- 

 tropic matter. 



In other corroded quartz-grains nests of anthrophyllite have 

 formed, as clusters of prismatic needles ; or we have beautiful little 

 deposits of andalusite crystals in elongated prismatic form, with the 

 dichroism unusually well shown. 



"^rurning now to the nodules which are cut in these sections, we 

 find points of great interest, which assist us in interpreting the 

 nature of the rock and the processes by which it has been altered. 

 The nodules are sharply defined, and are darker at the edges than 

 towards the centres. This darkness is due to concentration of 

 pigment, and also to the presence of some ferruginous colouring 

 in parts. In these nodules are large patches of perfectly clear, 

 transparent, almost colourless, and quite isotropic matter, containing 

 no rutiles or other enclosures. With high powers and with suitable 

 illumination, it may be seen that these patches are not quite 

 structureless. There is a minute granulation discernible, and in 

 some of them it passes to a fibrous condition. Other patches of 

 similar material contain large bundles and sheaves and spherulitic 

 aggregates of felspar-needles with a little quartz. ^ And inter- 



1 It used to be considered that in sheaves and spherulites of this description all 

 negative fibres were felspar and all positive fibres were quartz, but it has been 



