428 J. A. PHILLIPS ON THE CHEMICAL AND MINERALOGICAL 



indications of being altered crystals. An analysis, in duplicate, 

 of a freshly broken specimen from this locality, afforded the fol- 

 lowing results (sp. gr. 3*00) : — 



I. II. 



„ T , f hygroinetric *34 "37 



Water (combined 2-88 274 



Silica 46-73 46-51 



Carbonic anhydride trace trace 



Phosphoric anhydride trace trace 



Alumina 19*28 19/33 



Ferric oxide , 2*86 2"72 



Ferrous oxide 7'57 7*73 



Manganous oxide trace trace 



Lime 10-76 10-81 



Magnesia 6"98 7*15 



Potassa '38 '30 



Soda 1'96 2-08 



99-74 9974 



When thin sections are examined by transmitted light, they are 

 seen to consist of a colourless base containing a little granular 

 quartz and altered magnetite or titaniferous iron, through which 

 is thickly disseminated a crystalline greenish mineral, together with 

 a greyish dust-like substance frequently observed in altered crystal- 

 line rocks. In this mixture are enclosed numerous crystals of 

 uralite of various sizes, as well as a few less distinct forms, some 

 of which are pseudomorphs after plagioclase still occasionally re- 

 taining traces of characteristic striation. 



The outlines of the larger crystals of uralite are not usually well 

 defined, but have often been attacked by a species of metamorphism, 

 through the action of which they have become externally converted 

 into a greyish pulverulent substance, similar to that contained in 

 the base, by which their angles become gradually rounded and 

 finally disappear. By reflected light, this dust-like product is 

 greyish white in colour, and the more transparent portions of the 

 base appear milky. 



The appearance presented by crystals of uralite which have been 

 more or less deeply attacked, will be understood on reference to 

 Plate XIX. figs. 1, 2, and 3, which are after photographs, and are 

 magnified 17 diameters. Fig. 1 represents a twin uralitic crystal 

 which has undergone this change not only around its edges, but 

 also, to a less extent, along a line of crack which is diagonal to the 

 plane of twinning. In fig. 2 this alteration of the crystal has pene- 

 trated to a greater depth than in the former case, while fig. 3 is the 

 mere image of a replaced crystal. 



The illustrations given will probably be sufficient to render intel- 

 ligible what has been said on this subject ; but there would be no 

 difficulty in preparing, from the ten sections which have been made 

 of this rock, a series beginning with a crystal scarcely attacked on 

 its edges, and passing by imperceptible gradations to a mere dusky 

 shadow of a crystalline form. 



