14 Mr. J. A. Phillips on certain Slates, Felsites, and Elvanites 



frequently much coloured by peroxide of iron. A section of this 

 slate, made parallel to one of its lines of cleavage, when examined 

 under the microscope, was found to consist of an amorphous 

 matrix through which is somewhat thickly disseminated a floc- 

 culence of a dirty greenish colour, perhaps due to the presence 

 of minute quantities of chlorite. A few well-defined quartz- 

 crystals were also apparent. Its chemical composition was found 

 to be as follows : — 



I. II. 



Water* -80 -80 



Silica 74-36 74-19 



Alumina .... 7'66 7'90 



Peroxide of iron . . 7'00 7<20 



Protoxide of iron . . '90 -90 



Lime 1-12 104 



Magnesia .... '48 43 



Potassa 2-10 2' 



Soda 5-23 4-^ rt 



^•001 

 *-87 J 

 99-65 99-33 



Metamorphosed Slate?, spec. grav. = 2'65. — A section pre- 

 pared from a band, apparently of highly metamorphosed slate, 

 lying to the east of the foregoing, in which the cleavage-planes 

 had to a great extent become obliterated, was found under the 

 microscope to be chiefly composed of felspathic-lookiug crystals 

 crossing each other in all directions, with here and there some 

 minute scales of chlorite. These crystals, which readily depo- 

 larize polarized light, are nearly transparent; but the small 

 amount of potassa, soda, and lime present in the rock, as shown 

 by analysis, renders it improbable that so large a proportion of 

 it can consist of any variety of felspar. 



Its composition, as found by analysis, is as follows : — 



Water { 5'50 



Silica 52-66 



Alumina 18*33 



Peroxide of iron . . 10*43 



Protoxide of iron . . 9*00 



Lime 1-76 



Magnesia .... trace 



Potassa -73 



Soda 2-12 



100-53 



* Of which "34 was lost in the water-bath. 



t In the second analysis a small loss was experienced from spirting when 

 evaporating. 



X Of which '50 was lost in the water-bath. 



