PLATE XXXV. 



FIG. 1. 

 BIOTITE-GRANITE (GKANITITE.) 



SIIAP. 



Magnified 30 diameters. Drawn with polarizer only ; short axis right and left. 



The minerals represented are quartz (24), felspar (10), dark mica (9), 

 chlorite (12) and apatite. 



The dark mica occurs in hexagonal plates. In the upper part of the figure 

 to the right is a hexagonal section which has been more or less changed to 

 chlorite. The brown portions of this section give a uniaxial, or nearly uniaxial, 

 figure in convergent light ; the green portions do not give a distinct figure. 



The central part of the drawing is occupied by a section of biotite at 

 right-angles to the basal cleavage. It appears of a light brown tint because 

 the traces of the cleavage are nearly at right angles to the short axis of the 

 polarizer. The colourless spots in this and in the other sections are sections 

 of apatite. The section of biotite to the left is cut in the same direction as 

 the last, but is of a darker colour because the cleavage cracks make a smaller 

 angle with the vibration plane of the light (short axis of the polarizer). The 

 section in the lower part of the figure is also at right angles to the basal 

 plane. The dark parts are unaltered biotite ; the dark colour being due to 

 the intense absorption of the rays vibrating right and left and therefore 

 parallel with the cleavage cracks. 



The quartz and felspar are mostly devoid of crystalline form. In other 

 parts of the slide felspar (orthoclase) plays the role of matrix to the quartz ; 

 strings of felspar-substance lying between detached grains of quartz. 



Apatite occurs in well-crystallized hexagonal prisms and is especially 

 abundant as inclusions in the biotite. 



This granite is intrusive in Ordovician strata. 



FTCr. 2. 



ENSTATITE-DIOKITE. PENMAENMAWB. 

 Magnified BO diameters, crossed nicols. 



This figure shows felspar (10), quartz (24), rhombic pyroxene (2) and 

 iron ores. 



Plagioclase felspar occurs in idiomorphic crystals which give lath-shaped 

 sections. These sections show both multiple and binary twinning. Uu- 

 twinned felspar, probably orthoclase, occurs intergrown with quartz to form 

 micro-pegmatite. The rhombic pyroxene is not well represented in the 

 portion figured. It occurs in more or less idiomorphic crystals, but the 

 angles are frequently rounded. It is green in colour when viewed in ordinary 

 light and sometimes shows a fibrous structure. These features are probably 

 the result of alteration. It is interesting to note that this section is ground 

 to such a thickness that the maximum tint given by the quartz is the 

 yellow of the first order. Orthoclase, in sections of such thickness, never 

 polarizes in- colour in consequence of its slightly lower double-refraction. 



Micro-pegmatite and quartz play the role of ground-mass to the other 

 constituents. 



This rock was described by Mr. J. A. PHILLIPS (Q.J.G.S., Vol. XXXIII., 

 1877, p. 423). It is intrusive in Lower Palneozoic strata, 



