54 MICROSCOPIC STRUCTURE 



under my notice the tachylyte has, apparently, become inter- 

 woven into this spherulitic substance. 



It also occurs at Fern Hill, near Deddington, and, in a 

 lesser quantity, at Bumie. 



Microscopical Characters. 

 This is a basalt glassu yellowish brown and structureless, 

 cooitaining large opaque spheroidal segregations inert on 

 polarised light, and only capable of being examined towards 

 their edges, which, being thinner, transmit a little light. 

 In this part their colour is dark brown, becoming slightly 

 purple at the periphery, which is roughly crenulated. Over 

 their thinnest areas they may be seen under a J objective 

 to be crowded with globulites and thin rods, the latter 

 essentially trains of globulites forming longulites, and 

 arranged mostly radially towards the circumference. Their 

 arrangement side by side resembles that of hairs on the 

 coat of a furry animal. This structure ceases on approach- 

 ing the crenulate border. The smaller dark brown crenuli, 

 or segregations, in the rock are too dense to transmit light. 

 Many of them are surrounded by an absorption area, in 

 which the glass is bleached to a pale yellow, and incipient 

 areas of this description are scattered everywhere, giving the 

 field a somewhat mottled appearance. These spheroids 

 appear to be independent of the cracks in the glass, and 

 which pass through them undeflected, w^hich suggests that 

 these segregations were the latest phase in the consolidation 

 of the rock. The glass of which the ro'ck consists is 

 covered with a network of fractures, and trains of globulites 

 have occasionally collected along the cracks, which are also 

 frequently the depositories of minute granules of magnetite. 

 With a high power, globulites may be discerned in abund- 

 ance everywhere in the glass. 



jVo. 2. — TAmhurgite. 



(From Bumiet-Waratah Railway. Sp. gT., equal to 2 -8.) 

 This is a dense, hard, and extremely tough rock, so much 

 so that it became notorious during the construction of the 

 railway connecting Bumie and Waratah, where it occurs as 

 a narrow band at the 7-mile. It is dark, almost black, in 

 colour, and very fine-grained in texture. 



Microscopical Characters. 



This is a felspar-free basalt, with augite and olivine, equal 

 to limburgite (Rosenbusch) and magma-basalt (Boricky), 

 and has many of the features shown by slices of Bohemian 



