100 
dimensions of the quartz grains of this particular specimen 
are '5 mm., although elsewhere the rock passes into a coarse 
grit and conglomerate. The quartz is either perfectly clear 
or encloses interpositions, either central, scattered, or arrang- 
ed in bands. They are mainly unindividualised. Undulose 
extinction is apparent under crossed nicols. 
Felspar is very subordinate, or practically absent. 
The orginal planes of bedding are indistinctly discernible 
by a general tendency of the longer axes of the separate 
grains to arrange themselves in parallel lines. 
No trace of any organism has been preserved in the sec- 
tions observed, but a similar rock at Mount Watt is highly 
fossiliferous. 
Metamorphic Grit (Ordovician). 
Locality. — Ayers Rock, Northern Territory. 
Macroscopically. — A dark, metamorphic grit,* which on 
casual observation may be, and has been, mistaken for an 
eruptive rock, the large felspars showing up conspicuously 
from the mass with their surfaces of cleavage. The quartz 
grains are clearly recognised as derivative (''clastic" j. A 
black mica, ores of iron, and other foreign minerals are 
among the grains. The rock may be termed a greywacke 
(Grauwacke) or arkose. The aggregation of waterworn 
grains of quartz and felspar (one single grain of the rock, 
moreover, often consisting partly of quartz and partly of fel- 
spar, stilJ in juxtaposition as orginally in an igneous rock) 
suggests the disintegration of granite. 
Mic?'oscopically . — The rockf is compact and composed es- 
sentially of quartz and felspar (allothigenous), with addi- 
tional fragments and flakes of ores of iron and mica fauthi- 
genous). 
The quartz occurs as more or less irregularly rounded and 
rolled grains, containing numerous gaseous inclusions in bands 
and streaks, or scattered. Some of the grains, moreover, ex- 
hibit a microscopic intergrowth between quartz and felspar. 
The felspar is of several species. A typical microcline pre- 
dominates, and is often traversed by narrow parallel streaks 
of strongly doubly refracting altered mineral (kaolin). Micro- 
graphic intergrowths of this felspar, with quartz, appear to 
be prominent, although the effect is masked. ''Strain sha- 
dows" under crossed nicols. 
* ''The rock is a very indurated, and, to some extent, altered, 
arkose saaidstone, decidedly gritty in parts." — Tate and Wott-. 
Rep. Horn Exped. Centr. Anstr., Phys. Geog.. page 8. 
t A description of a similar rock, by Messrs. Smeeth and 
Watt, has appeared in the report of the Horn Expedition, Petro- 
logy. "Arkose"; No. 213, page 83. 
