THE MICROSCOPIC STRUCTURE OF SOME 

 TASMANIxiN ROCKS. 



By W. F. Petterd. 



I PURPOSE to describe in this paper some aberrant members 

 of the basalt family, which, although not common in this 

 Island, are occasionally met with, and which are not only 

 difficult of interpretation to the ordinary observer, but are 

 sometimes a puzzle to the field geologist. 



As is so well understood, the normal basalts are basic lavas 

 (silica = 45 to 55%) of high specific gravity and dark colour, 

 and are essentially composed of plagioclastic (labradorito), 

 felspar, augite, magnetite, and often olivine. They cover 

 considerable areas in the northern part of the Island, and 

 isolated patches occur in the eastern and southern portions. 



As far as is known, the Tertiary basalts as occurring here 

 do not differ in their normal characteristics from the 

 familiar types, with the exception of the fayalite basalt of 

 One-Tree Point, and the melilite basalt of the Shannon Tier. 



The varieties now enumerated and described are the ab- 

 normal accompaniments of the usual types which occur in 

 but limited quantity, or are formed under peculiar local 

 conditions. 



No. 1. — Tachylyte, Bothwell. 



(Sp. Gr. : Spherulitic, equal to 2 • 72 ; non-spherulitic, 

 equal to 2 • 77.) 



This is the glassy form of basalt, originating from the 

 rapid cooling of the magma by contact with a cooler sub- 

 stance. It is commonly in thin selvage layers, but some- 

 times is met with, as at the locality quoted, in comparatively 

 large lumps. It varies in colour from rich dark brown to 

 intensely black, and when freshly broken has a shining 

 vitreous lustre. It is sub-conchoidal in fracture, and, 

 though hard, it is brittle. On weathering it often generates 

 a thin film on the exposed surface of a beautiful pale to 

 dark ultramarine blue, which renders it an object of 

 curiosity and interest. External nodular spheruloids are 

 occasionally prominent on the surface, which show a pro- 

 nounced radiating structure. It may vary to a structure 

 known as variolite, and in a single example which has come 



