88 HORN EXPEDITION — PETROLOGY. 



strong pleoclirosiii) of the liorixl)leiulo. Tlio rest of tlu^ rock is coinposed of clear 

 granular plagioclase, with apparently high extinction angles. Tlie hornblende fills 

 in accurately the spaces between the granules of felspar, while a number of small 

 felspar grains are completely included in some of the hornblende crystals. In the 

 ma.ss the rock appears to have been subjected to a certain amount of crushing. 

 This rock occurs as an intrusive dyke in the McDonnell Ranges, near Fish Water 

 Hole, on Ellery Creek. 



Diopite. No. 19. Plate I., Fig. 5, and Plate IV., Figs. 5^, 6a. 

 Sp. gr. 2-99. 



The essential constitutents of this rock are hornblende and plagiocla,se. The 

 felspar is clear and fresh, but shows .signs of crushing, the twin lamellai being in 

 places discontinuous and bent (Plate IV., Fig. 6a). Two sets of twinning are 

 often to be seen, and a series of solution cavities appear frequently to be developed, 

 the longer a.xes of which lie parallel to the trace of the plane 010. Occasionally 

 these cavities have a dendritic form, and are filled with a yellowish ferruginous 

 material. Compact greenish-brown hornl)leude is present in considerable quantity 

 in large, clear and fresh grains with very irregular outlines, being formed 

 subsequently to the majority of the felspars. Closely associated with these are 

 numerous grains of a very pale green augite (Plate I., Fig. 5), shown by 

 stippling, most of which show in places transition into hornblende (Plate IV., Fig. 

 5/^), which is identical in character with the rest of tiie hornblende. 



This suggests the possibility of much of the hornblende having been formed 

 at the expense of augite. This view is supported by the fact that the augite 

 grains have suffered considerably in the crush, while the grains of hornblende show 

 little cracking or dislocation. At present the hornblende is in considerable excess 

 of the augite, and in fact appears to form about one-half of the whole mass of the 

 rock. Inclusions of felspar are common in both the grains of augite and horn- 

 blende. In a portion of one slide a great number of granular zircons lie included 

 in the hornblendes ; a few of them are present also in the felspars. This rock 

 occurs at Mount Brassey, in the Hart Range. 



GabbPO. Nos. 109 and 90. Plate II., Fig. 3. 



In thin sections this rock is seen to be a granular aggregate of crystals of 

 diallage, plagioclase and magnetite, with a small quantity of greenish decompo- 

 sition products. Although the pyroxene exhibits more or less well-developed 



