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prepared. In such sections orthoclase never shows any colour under 

 crossed nicols. It polarizes always in neutral tints. Where micro- 

 pegmatite occurs in the rock it always plays the role of ground-mass. 

 The mother-liquor, left after the separation ot the plagioclase, the 

 ferro-magnesian constituents and the iron-ores, crystallized as micro- 

 pegmatite exactly as in the rocks of Groby and Markfield. 



Of the ferro-magnesian constituents enstatite (ferriferous) is by far 

 the most important. It is usually represented by green fibrous pseudo- 

 morphs (biotite) but in some of the least altered rocks it still preserves its 

 original character. It is then seen to possess the pleochroism of hyper- 

 sthene. Longitudinal sections are brown or reddish-brown for rays vibrating 

 at right angles to the principal axis and green for rays vibrating parallel to 

 this axis. Crystalline forms may be observed but the angles are as a rule 

 somewhat rounded. The forms in the prismatic zone are (100), (110) and 

 (010) as in the enstatite-andesites and the pinacoidal forms are developed 

 at the expense of those of the prism. The terminal faces, so far as they 

 can be determined, appear to be the flat domes characteristic of the 

 rhombic pyroxenes. Longitudinal sections of the altered crystals are 

 pleochroic in shades 01 green the greatest absorption occurring when 

 the rays vibrate parallel with the vertical axis. It is worthy of note that 

 the double-refraction of the green crystals is higher than is common in the 

 rhombic pyroxenes. 



Augite, which is almost always present, is allotriomorphic. It is very 

 pale brown and devoid of pleochroism. Twinning according to the 

 ordinary law is very common and the sections not unfrequently show the 

 diallagic striation parallel to the basal plane. In clino-pinacoidal sections 

 of twinned individuals this striation gives rise to what may be termed the 

 herring-bone structure. The augite is often fresh Avhen the enstatite has 

 been entirely replaced by the green fibrous mineral. Biotite occurs very 

 sparingly in the form of ragged plates (ophitic) often penetrated by the 

 lath-shaped felspars. In its fresh condition it is very strongly dichroic; the 

 colour for rays vibrating parallel to the principal cleavage being a rich deep 

 brown It passes into green chloritic alteration products. Iron-ores are 

 present. Sometimes they occur in ragged plates or skeletons, suggestive 

 of ilmenite. Apatite is present in long slender prisms and is sometimes 

 very abundant in local patches. The quartz often contains liquid cavities 

 with moving bubbles. In the more altered rocks the felspars are replaced 

 by greyish granular matter giving aggregate polarization. 



The normal rock is of medium grain and of a greenish grey colour. 

 Irregularly distributed through it are certain light grey veins and patches 

 These have been specially investigated by Mr. WALLER. They differ in 

 chemical and mineralogical composition from the main mass of the rock in 

 a very striking and suggestive manner. They contain about 7 p. c. more 

 silica and about 1*5 p. c. more alkali. Moreover the potash is in 

 excess of soda in the veins, whereas the reverse relation holds in the rock 

 itself. The ferro-magnesian constituents are less abundant in the veins 

 than in the rock. Ilmenite, more or less changed to leucoxene, and 



