79 
well-defined planes, striking W. 25° N., with a northerly 
dip, and, less conspicuously, in planes striking N. 3"^ E., 
with a dip of 75° W. Secondary minerals line the walls of 
these joints, along which, moreover, slight faults and hitches 
have occurred. 
The Indulkana Outcrop. 
About twelve miles east of Indulkana Sprang, adjacent to 
Chambers's old wagon track, a small exposure of bedrock 
exists, and, whilst not many square miles in extent, indica- 
tions are not wanting that the rock may be found at no great 
depth over a much wider area. The exposure is 1,300 feet 
above sea level, and is surrounded on all sides by a capping of 
"desert sandstone''' barely exceeding 30 feet in thickness. 
Igneous Bocks. — The intrusive rocks are of the acid and 
intermediate families. Diorite dykes predominate, though it 
is often difficult to determine the exact planes of contact with 
the intruded schists on account of the severe shattering of 
the rock. At least four major diorite intrusions have occur- 
red in direction east and west, with slight variations, due 
possibly to subsequent earth movement. The largest mea- 
sures one hundred yards in breadth. In places where the con- 
tact with the schist is visible the latter rock appears baked 
and highly schistose, with upturned planes of schistosity. The 
diorite is for the most part fine-textured, quartzless, and 
micaceous : on the surface the rock is usually '"honeycombed" 
by unequal weathering of the constituent minerals, the libe- 
rated iron oxides coating the surface with a "rust." 
Intrusions of graphic granite, pegmatite, and greisen 
have occurred previous to that of the diorite. This is evident 
from the fact that the diorite dykes are often found cutting 
the pegmatite, the latter having thereby frequently suffered 
lateral displacement. The mineralogical character of these 
acid rocks varies considerably. Their common feature is 
coarse crystallisation of the constituents. In some dykes 
quartz predominates, in others it is subordinate to felspar, 
while mica occurs as irregular aggregates in the greisen and 
occasionally as an accessory in the pegmatite — in the latter 
case usually in a state of partial decomposition. On the 
western limits of the exposure igneous intrusion is marked 
by dykes of graphic granite and schorlaceous greisen, the lat- 
ter including large, perfect crystals of black tourmaline and a 
light-coloured microline._ The general direction of intrusion 
is east and west, although dykes may be found running at 
right angles to this. True granite is feebly represented by a 
coarsely crystalline rock, with pink crystals of orthoclase, 
rather subordinate quartz of a bhiisli sub-opaline character 
and a greenish biotite. 
