90 



essential that they be cut perpendicular to the twin lamellae. 

 The maximum angle ascertained is the one to be taken, and 

 this in the present instance is 42°. The felspar therefore 

 belongs to the labradorite-anorthite group. The individuals 

 polarise in the neutral tints of the first order of Newton's 

 scale. At their terminations they are frequently turbid, and 

 this turbidity makes it difficult to say whether there is or 

 is not any small occurrence of glassy base between the ends 

 of some of the prisms. There are reds of apatite in a few 

 of the crystals, and inclusions — sometimes fluid cavities — are 

 numerous. 



ATJGITE. 



This is in groups of large ill-defined crystals or plates 

 with irregular boundaries, moulding themselves, as it were, 

 upon the felspars, and at times enclosing them, after the type 

 known as ophitic. This structure is prevalent in gabbros, 

 so-called diabases, and dolerites, and indicates that the augite 

 crystallised out later than the felspar, which it wraps round 

 and sometimes wholly encloses. 



The ordinary order of crystallisation is now understood to 

 be (1) iron ores, etc., (2) the ferro-magnesian crystals 

 (olivine, augite, hornblende, iDiotite), (3) felspars, (4) quartz. 

 But in these ophitic rocks the felspars precede the augite ; 

 accordingly they have the most perfect symmetry of form, 

 having been free to crystallise without interference from 

 adjacent crystals. The large masses of augite, on this 

 assumption, are not analogous to lava phenocrysts brought up 

 to the surface ready made, often corroded, dislocated, and 

 senile. Some of them measure ^^in. in length, and though 

 their component crystals are far from being idiomorphic, a 

 few, out of the prism zone, are twinned, and have the modifica- 

 tion of zonal structure known as the hour-glass form. The 

 augite is monoclinic and non-pleochroic, colourless to light 

 brown. Extinction angle 41° to 50°. On the whole they are 

 not fresh, decomposition processes having commenced, and in 

 some plates schillerisation has set in. 



Iron oxide (magnetite) occurs in scattered grains, but is 

 not abundant, the iron having apparently been used up in the 

 augite, which from the marginal colouring we may infer to be 

 highly ferriferous. 



Yiridite or chlorite exists as a greenish, radiately fibrous 

 decomposition product, derived probably from disintegration 

 of the augite. Its strong pleochroism helps to discriminate 

 it from serpentinous matter. It evidently belongs to the 

 chloritic group, and gives rise to the name greenstone. 



There is no base actually determinable as such. The 

 felspar crystals abut closely upon each other, as can be 

 clearly seen by employing a quarter-inch objective. 



