F. P. Mennell — Wood's Point Dyhe, Victoria, Australia. 393 



That it is original in the present instance can scarcely be doubted, 

 despite the occurrence of the pyroxenes. The prism angles, where 

 seen, are those proper to the species, and there are no traces of the 

 wedge-sliaped outgrowths or fibrous structure so characteristic of 

 the uralitic varieties. The pleochroism is striking, giving deep 

 yellow brown for c to nearly colourless for a, while the interference 

 tints are frequently to some extent masked by the strong absorption. 

 The extinction angle in longitudinal sections reaches a maximum of 

 18°, while a twin in which the sharp boundary of the individuals 

 showed it was cut parallel to the clinopiuacoid, gave 13° on each 

 side of the composition plane. 



The pyroxenes are distinguished from the hornblende by their 

 cleavage and extinction angles and their want of colour. The 

 augite shows distinctly stronger refraction and double refraction 

 than the hornblende, while the enstatite gives low interference tints 

 and shows the straight extinction in several longitudinal sections. 

 The augite is sometimes surrounded by hornblende ; it may, however, 

 be regarded as an inclusion in the latter rather than as intergrowu 

 with it. 



Fig. 1. — Hornblende Pi)rpliyi'ito. Crossed nicols, x 50 diam. 

 The felspar does not all belong to the same period of consolidation ; 

 indeed, it seems to have been developed in at least three distinct 

 generations or regenerations. The largest crystals, those which 

 are idiomorphic towards the hornblende, belong to the first period, 

 smaller crystals and microlites represent the second generation, 

 while other small crystals, much the freshest constituents of the 

 rock, must be regarded as in all probability of secondary origin. 

 The first class have undergone decomposition to such an extent that 

 they are nearly opaque. They appear, however, to extinguish almost 

 straight, and unless this is an efi"ect of decomposition they would 

 seem referable to oligoclase. The second series are somewhat 

 fresher and exhibit both the Carlsbad and Albite types of twinning, 

 though the lamellEe in the latter case are never numerous. A few 



