STRUCTURE AND COMPOSITION OF A BASALT FROM BONDI. 229 



The rapidly gelatinising character of the rock may also be seen 

 "by reducing a gram to fine powder, and covering it with about 

 ~twice its own volume of hydrochloric acid in a test tube. The 

 mass will set solid in the test tube. 



A polished slab of the basalt shows the effect of hydrochloric 

 acid rapidly, the black of the polished surface altering to a light 

 grey uniformly. If it was a zeolitic product, such as analcime, in 

 the slab that yielded so readily to the acid, the grey and affected 

 portion would most likely show in patches, whereas we find the 

 whole surface attacked around the porphyritic and insoluble con- 

 stituents. The mineral so affected is not a zeolitic product, but 

 •rather a constituent very evenly distributed through the rock. 



I have cut thirty thin slices of the basalt. Under the micro- 

 scope they vary very little in structure. A description of two 

 slides will be fairly descriptive of the microscopic structure of 

 the whole. 



Slide No. 206. — Micro-porphyritic structure, pronounced 

 olivine forms the largest individuals, measuring up to the one- 

 fiftieth of an inch. They are for the most part only portions of 

 larger crystals ; some show a few idiomorphic faces, the others 

 being lost by corrosion of the liquid magma. The fractured and 

 broken condition of the larger olivines is due to mechanical causes 

 that acted while the rock was yet plastic. They contain inclus- 

 ions of the ground mass, and in two instances are broken in parts, 

 and wedge-shaped masses of the base penetrate the crystals to their 

 centres. Blades of brown mica occur interstitially. The augites 

 show a few good examples of zoned structure, and lines of secondary 

 growth, which latter are marked by lines of magnetite crystals 

 along the former faces. There is a remarkable mosaic aggregate 

 of augite microlites, the whole surrounded by a line of magnetite 

 grains. The triclinic felspar laths are seen lying along the faces 

 of micro-porphyritic ingredients in a rudely parallel fashion. This 

 does not amount in character to a decided "flow" structure. 

 The feldspar laths measure on an average the one-hundredth of 

 an inch in length. The undetermined six-sided mineral already 



