374 Transactions. — Geology. 



crystallize before the olivine. A few small grains of olivine 

 also occur in the groundmass. 



The augite is not abundant ; it is mostly in the form of 

 small irregular grains in the groundmass, but a few large 

 crystals occur (2, rock-section 70, PI. XXXV.). In this particu- 

 lar section these crystals are not well developed, but in some 

 sections of the basalt belonging to this class an occasional very 

 regular augite crystal occurs. 



The feldspar is in the form of small crystals, giving the 

 usual lath-shaped sections. They are all minute and approxi- 

 mately equal. There are no microporphyritic crystals of this 

 mineral present. 



A half-developed flow-structure is observable in this sec- 

 tion. The greater number of the olivine crystals and the 

 feldspars are arranged with their long , axes pointing in one 

 direction (the direction of flow), and the groundmass separates 

 into two currents at the porphyritic crystals, flows past 

 them, and unites beyond. This structure is of somewhat 

 frequent occurrence in the basalt of this class ; also, though to 

 a less marked degree, in the basalt of class 2; but not in 

 that of class 3, where the lava had ceased to flow at all rapidly 

 before the rock began to solidify. 



2. Basalt which has cooled less rapidly than that of class 1, 

 but yet in most cases while the lava v^as still in motion. — 

 This class includes all those varieties in \Yhich, as in the pre- 

 ceding class, there is a well-defined base between the larger 

 crystals, but it is more crystalline and less glassy than in 

 I'ocks included in that class. These basalts, being interme- 

 diate between, show greater variety than, the basalts of the 

 other two classes, but rock-section 17 (PI. XXXV.) is fairly 

 representative of class 2. It illustrates the large amount of 

 augite frequently occurring in the groundmass of rocks belonging 

 to this division. A glance at the illustration will suffice to show 

 the smaller proportion of magnetite and glassy groundmass in 

 this as compared with the preceding section. 



The olivine is similar to that just described. The augite 

 in this section is of a pale yellowish-purple tint, slightly di- 

 chroic, and is mostly in the form of rounded grains or plates of 

 irregular shape. Some of the feldspar laths reach a fair size, 

 and in some of the rocks of this class {e.g., sections of basalt 

 from One-tree Hill and the Three Kings Hills) are very regu- 

 lar and perfect in outline. 



3. Basalt which has cooled comparatively slowly — that is, 

 which has crystallized near the centre or bottom of the 

 stream. — The olivine occurs mainly micro-porphyritically, its 

 crystals being relatively smaller than in the preceding classes. 

 The ingredients are more equal in size, and there is but little 

 glassy or semi- vitreous groundmass ; the structure is, in fact, 



