THE GEOLOGY OF MITTAGONG. 517 
felspar. It covers an area of 450 acres, and is surrounded 
by tilted sandstones on the northern side, while the east 
slope is overlaid by later basalts. The stone is largely 
used for building purposes and is obtained from seven 
quarries on the steep southern and western faces. Of 
these the most eastern (Loveridge’s) is the largest. The 
rock is of very uniform character though the stone from 
Saunder’s Quarry nearer Mittagong is of darker hue than 
that further east. Probably the lighter colour of the latter 
is part due to its proximity to the Hawkesbury Sandstone. 
It isremarkably free from fissures and cross cracks, and so 
it is possible to obtain huge blocks for architectural pur- 
poses. Narrow segregation veins of beautiful sanidine 
(glassy orthoclase), mingled with large hornblendes and 
eegerine crystals, traverse the syenite. Small irregular 
grains of fluorspar are fairly common in Loveridge’s quarry, 
while sparkling little quartz geodes can be found in western 
quarries. Small black fragments and stains of carbonaceous 
material are present. ‘These are supposed to be due to the 
condensation of bituminous matter carried up by volcanic 
rock, in a gaseous state, from the underlying coal measures. 
The Gib represents the denuded plug of an old volcano. 
It is improbable that it represents a denuded laccolite> 
since only a very few hundred feet of shales, judging from 
surrounding strata, could have remained undisturbed to 
check the immense volcanic energy of so large a mass of 
molten magma.—(See Section IV. No.1.) Again although 
any ejectamenta which may be present on the eastern slope 
have been covered by basalt, and much have undoubtedly 
been removed by denudation, yet what appears to be an 
ejected breccia occurs at the foot of the western slope. 
There is also a true trachytic lava which probably welled 
up from a parasitic cone, but this is described more fully 
later on. (See 1. c. infra). 
