64 



enstatite, diallage, and chromite. The pyroxenes are slightly 

 idiomorphic, moderately large, and embedded in a granular 

 mass of olivine, thus presenting a pseudo-porphyritic 

 structure. 



The olivine is by far the most abundant mineral. There 

 is an indication of the (001) cleavage in some sections. A fair 

 amount of cracking has gone on, with little or no decom- 

 position to serpentine, but in some instances slight brownish 

 stains of ferric oxide are seen ; this, however, is visible only in 

 the vicinity of a few microscopic grains of magnetite. A few- 

 liquid inclusions are present. In order of abundance the 

 rhombic pyroxene, enstatite, follows the olivine. It is light - 

 brownish-green in colour, slightly idiomorphic, enwrapping a 

 few grains of olivine and diallage in a pcecilitic manner (plate 

 xix., figs. 1 and 2). The (110) cleavage is well developed, 

 being, in some instances, broken up into parallel plates along 

 this parting. The basal sections show two good pinacoidal 

 cleavages intersecting at 88°, together with a less perfect 

 cleavage at 45°. Decomposition to bastite is common, especi- 

 ally in the vicinity of cracks (plate xix., figs. 2 and 3). 

 Columnar structure is well developed. In crossed nicols good 

 lamella twinning is seen to advantage (plate xix., fig. 4), and 

 in some cases the lamellae may be seen in ordinary light. 



The enstatite and diallage afford a good instance of 

 lamella intergrowths with one another (plate xix., figs. 2 and 

 4). When the enstatite is in the position of extinction the 

 diallage, in the same combination, is extinguished at only 

 39° from this position. In convergent polarized light a lare;e 

 optical axial angle is seen. The sign is positive, and the 

 dispersion p < v, which is an indication that the mineral is 

 low in iron. A few liquid and magnetite inclusions are 

 present. 



The diallage, which occurs in irregular grains of a light - 

 green colour, is probably chrome-bearing. Some sections 

 show two fair pinacoidal cleavages intersecting at 89°, whilst 

 others show a, good prismatic cleavage. The extinction, which 

 is 39°, is the chief distinguishing character from the enstatite. 

 Decomposition is rarely seen in most sections, but in sections 

 near the tachylytic coating the diallage is, to a considerable- 

 extent, decomposed to chlorite and epidote. In the undecom- 

 posed sections liquid inclusions are seen. The chromite is 

 present in small rounded or irregular brown grains. 



The minerals in contact with the tachylytic coating are 

 partially absorbed by it. Leucoxinization has gone on to a 

 considerable extent just within the junction of the tachylyte, 

 proving that the magnetite present is titaniferous. 



