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THE AUSTRALIAN NATURALIST. 111 
GEOLOGICAL EXCURSION TO BONDI. 
June 14th, 1919.. 
Leader: Miss A. Brewster. 
The following is a summary of the more important 
Pe demonstrated :— 
The bay shows an attack by erosion along a line 
of aia 
2. Inter-bedded shale is exposed in the tramway 
cutting just before the beach is reached. 
The headlands being more resistant, are less at-' 
tacked. 
4. The beaches composed of worn-down material 
sass by streams and spread out by wave action. 
. Ripple marks on the sand and structure of sand 
dries indicating direction of prevailing winds. 
6. The retarding action on sand movement of the 
Spinifex and other binders, and further back the plants 
acting as sand retainers. 
7. Note sandstone consisting chiefly of grains of 
quartz coated with tiny crystals of secondary quartz, 
giving them a glistening appearance. Oxide of iron 
and minute crystals of iron pyrites are also present, and 
in some beds decomposing felspar and small particles of 
graphite. 
8. Grits and conglomerates containing numerous 
pebbles of milky or reef quartz. These and the quartz © 
sandstone grains show that the beds were formed origin- 
ally from igneous rocks such as granite. 
9. Note vertical joints in the cliffs with resulting 
clean-cut faces. These and the bedding planes con- 
stitute lines of weakness to weathering action. 
10. The false and current bedding in the cliffs sug- 
gest shallow water conditions when the beds were laid 
down. 
Prominent dyke fissures, one running N. and S. 
the others E. and W. The N. and S. dyke cuts right 
across the cliffs, and, being the means of attack from 
behind, dooms the present cliff face. The E. and W. 
dykes form little inlets, and have a marked effect on the 
local coast contour. 
