294 J. A. THOMSON. 



East " are probably superficial deposits of desert origin 

 similar to the surface quartzites of the South African arid 

 regions. Dr. J. M. Maclaren has informed me that he has 

 found such terraces northwards from Leonora, and that he 

 considers them of analogous origin to the ferruginous 

 laterites of Western Australia, and proposes to designate 

 them by the name of "siliceous laterites." 



The only rock in the collection from the southern end of 

 this gneiss belt is that from Praser's Range. It is practi- 

 cally identical with a rock collected by Gibson from Simon's 

 Hill, Fraser's Range, and labelled "gneiss" in the Register 

 of the Geological Survey of Western Australia (No. 8696). 

 Besides garnetiferous gneiss, garnetiferous mica schist and 

 pegmatite from Fraser's Range. Streich records a horn- 

 blendic schist as forming the main mass of the range, of 

 which rock Stelzner writes: — "671 is according to the 

 microscopic examination of the rock section, an undecom- 

 posed diabase, which is distinguished on account of its 

 containing highly pleochroic augite and biotite and apatite 

 as accessory components." This is obviously the rock now 

 to be described. 



The hand specimen is a dark, distinctly banded rock, the 

 banding being due to the separation of the felspathic and 

 the femic minerals into poorly defined layers. Microscopic 

 examination shows that the latter minerals consist pre- 

 ponderatingly of hypersthene, with subordinate biotite and 

 rare hornblende. Iron ores and apatite are the only 

 accessories (Fig. 1, Plate XIV). The felspars are some- 

 times twinned on the albite and pericline laws, but the 

 twinning is fine and not very constant, and is absent in 

 many of the crystals. They all possess refractive indices 

 superior to that of Canada Balsam, so may be all referred 

 to plagioclase. The largest extinctions on symmetrically 

 placed albite lamellae amount to 19°, indicating a species 

 at least as basic as andesine. The felspars never show 



