122 WHITE : OATLAND COMPLEX OF IGNEOUS ROCKS. 
2. BASIC TYPE. 
(Plates IX., X.) 
It will assist in the description of the intermediate type- 
of rock, and save repetition, if the mineralogical features of the 
most basic type are next considered. The rocks in this section 
grade from hornblende-augite-diorites to a p\Toxenite. They 
are all characterised by the absence of original quartz except in 
those parts in which there has been an intrusion of this mineral 
as veins or threads from the more acid magma. In such cases 
it has had a noteworthy effect in altering the condition of the 
invaded rock at the planes of junction. 
Felspar is practically undistinguishable in the normal rock 
of this type, but in its place appears an allotriomorphic aggregate 
of mineral substance " undetermined " in the Memoir. The- 
aggregate is of a dusty greenish-brown or a faint turbid brown 
colour. Under crossed nicols, owing to its intimate mixture with 
chloritised hornblende, it sometimes shows an indigo tint. The 
mineral has a high refractive index w^hich, how^ever, is less than 
that of either of the hornblendes and the augite. Where the 
granules were larger they Avere determined as epidote, zoisite 
and quartz. In a few cases where the aggregates are completely 
enclosed by brown hornblende they show good outlines and 
suggest that they are the products of an extremely decomposed 
plagioclase felspar. In confirmation of this, the quantity of 
the minerals present in the aggregates, estimated as plagioclase, 
was determined, and it was found that the sequence in the three 
facies then became intelligible. Moreover, the order of crystal- 
lisation relatively synchronised with the order as more certainly ob- 
served in the intermediate and acid types. Its original felspathic 
character was placed beyond doubt by the phenomena observed 
in a section which showed this rock invaded by a single small 
vein of quartz. Near this quartz the dusty mineral was found 
associated with fresh felspar substance. Still nearer the quartz 
the dusty mineral disappeared and perfectly fresh felspars were 
alone found. The extinction angles of these clear felspars were 
measured and varied from 18 degrees to 20 degrees. The original 
felspar was, therefore, probably an andesine. 
