of the Bud tan District. 20 



(d) Very numerous needles and prisms of apatite. 



This ground-mass is decidedly of a micro-crystalline 

 character, but in it are small porphyritic crystals of — 



(a) Triclinic felspars, in which the brachypinacoid 

 predominates, producing tabular crystals. I could not obtain 

 any reliable optical measurements. 



In some slices these felspars were much altered to aggre- 

 gates of flakes, apparently micaceous. This was not the 

 case in the rock sample which I selected for analysis. 



(6) Crystals and crystalline grains, singly or in clusters, 

 of an almost colourless augite. Twinning frequent, accord- 

 ing to the ordinary law, composition face being ccP c» (100). 



(c) Crystals of magnetite. 



(d) Spaces filled by secondary products, usually quartz or 

 agate, but also calcite, and more rarely the minute green 

 siliceous amygdules which I have already mentioned in 

 speaking of the Diabase porphyrite. Chloritic minerals 

 (viridite) are rare, and, in fact, the principal pseudomorphs 

 appear to be calcite after augite, and ferric hydrate after 

 magnetite. 



The miscroscopic examination of this rock shows many 

 points in common between it and the Diabase porphyrite of 

 the Snowy River, and strongly suggests that it is merely a 

 somewhat different form of the same igneous rock. 



In order to make further comparisons, I prepared slices 

 from samples of an extremely hard and compact rock which 

 underlies the Devonian limestone at the junction of the 

 Buchan and Murendel rivers. 



On examining it under the microscope, I found its structure 

 to be completely micro -crystalline, being composed of a 

 perfect network of microscopic felspar prisms, rilled in by 

 augite grains and magnetite. In this ground-mass were a 

 few larger triclinic felspars, and rather more numerous 

 crystals of augite, so that, in fact, the structure of this rock 

 approached the micro-porphyritic ; the porphyritic mineral 

 being mostly augite. 



In this rock, a number of the augite crystals had been 

 removed, and the spaces filled by brown iron ore, which is 

 also common in the slice, and is often connected with 

 remains of magnetite. 



The following quantitative analysis is of the sample 

 selected for examination. The indicative ratio of this rock 

 is smaller than that of the Diabase porphyrite. In calcu- 



