1080 NOTES AND EXHIBITS. 



grains, so as to give the rock the aspect of having a perlitio 

 structure in places. tSome of these cracks are partly filled with 

 lines of globulites (1). These have the appearance of strings of 

 miscroscopic beads. The material shows a feeble double refraction. 

 It is uncertain whether these are globulites or a succession of minute 

 bubble-holes. If the latter they may be compared to the "bubble 

 ropes " so frequently observable in the Melbourne " blue-stone." 

 The latter are frequently 2 to 3 inches wide, and many yards in 

 length, and the bubble-holes about ^ of an inch in diameter. 

 ISpherulites are tolerably abundant. These consist of concentric 

 rings of clear and greenish glassy material partly devitrified, 

 the nucleus assuming a fibrous radial structure. A small 

 central cavity is often observable. Some of the spherulites are 

 separated, by sharp lines, from the surrounding base, but others 

 merge into it. It is difficult to determine whether their 

 growth was concretionary or incretionary, i.e., whether the 

 spherulitic structure commenced from the centre and spread 

 outwards, or whether it commenced from the sides of steam-holes 

 and extended itself inwards until the cavity was almost filled. 

 The sharp line of demarcation between the outer edges of some 

 of the spherulites and the magma argues an incretionary origin 

 similar to that of an agate, while in other cases the gradual 

 transition from the magma into the spherulite favours the suppo- 

 sition that their origin was concretionary and comparable to that 

 of spherulites in obsidian, and of concretions in orbicular diorite. 

 It is a remarkable fact that magnetite is almost wholly absent 

 from the base, being visible only as dusty zones encircling small 

 felspar crystals, and as dendrites lining cracks in the glass, and the 

 in sides of gas-pores in the olivine grains. This accounts for the 

 powder of this rock not being magnetic, in which respect it differs 

 from most of the European tachylytes. The iron is probably 

 combined with the silica as in the Hawaiian lavas. The augite 

 occurs in fragmentary and corroded crystals enclosing magnetite, 

 and in small imperfectly formed microscopic crystals of a 

 yellowish or purplish-brown colour, bounded externally by narrow 

 dark zones. The felspars are almost without exception triclinic 



