Rocks of the Howqua River. 47 



•>augite remains, but most of it has changed to secondary horn- 

 blende. Chlorite is abundant, ilmenite altered to sphene is 

 prominent, porphyritic crystals of albite or albite-oligoclase, one 

 ■or two prismatic crystals of apatite, and some relatively large 

 irregular quartz crystals occur. The background is a beautiful 

 .micrographic int^rgrowth of quartz and albite, the latter showing 

 lamellar twinning in places, and a refractive index lower than 

 quartz. The micrographic background and the larger quartzes 

 both appear to be of secondary origin. 



No. 108. — Diabase from spur south of Barney's Creek, Upper 

 Lick Hole Creek Valley, at south-east end of main diabase mass. 

 A diabase of moderate grain size, in which very complete secondary 

 alterations of the original minerals, augite, felspar and ilmenite 

 have occurred. A little original grey broAvn augite remains, but 

 most of it has been changed to secondary hornblende, some of it 

 prismatic to tabular, some to fibrous actinolite, some to vivid 

 green chlorite in clusters of radiating fibres. Quartz occurs in 

 scattered granules, and in micrographic intergrowth with augite 

 and with chlorite. This quartz may be primary or secondary. 

 Much secondary quartz occurs, more or less completely replacing 

 the lath-shaped felspars which, however, still show the outlines of 

 the crystals and the positions of the twin lamellae in a remark- 

 able way. An irregular vein, about \ inch wide, traverses the rock, 

 .and consists partly of quartz, but mainly of colourless to cloudy 

 prismatic and radiating crystals of lawsonite, a hydrated silicate 

 of lime and almuminium. Ihe mineral is recognised by its posi- 

 tive sign, and biaxial character, its high refraction and polariza- 

 tion colours up to second order, whereas the quartz shows low 

 neutral tints. The lawsonite is a secondary mineral derived mainly 

 from the alteration of the felspar, but in places is seen to develop 

 from altering ferromagnesian minerals in the mass of the rock. 

 Ilmenite is partially altered to leucoxene, and a little secondary 

 calcite and epidote are also present. No secondary albite was 

 recognised in the rock, and the original felspars are so altered 

 either by becoming cloudy, through incipient saussuritization or 

 replaced by quartz, that their original character is unrecognizable. 



No. 109. — Diabase from same locality as No. 108. A moderately 

 eoarse grained rock which is free from quartz, but otherwise has 

 suffered much the same changes as has No. 108. The large platy 

 albite or albite-oligoclase felspais have chlorite inclusions. Some 

 augite has developed a diallagic structure, and there is a consider- 



