18 C. HEDLEY. 



of the shore exercise considerable influence on the flora 

 and fauna that inhabit it. One geological outcrop produces 

 a broken beach of pools and crags, another a smooth flat. 

 Loose rocks that roll about and crush are a danger to the 

 marine community, while large immovable boulders afford 

 a long lease of shelter to their lodgers. The rough beach 

 presented by limestone supports a rich fauna. Algae are 

 favoured by the harsh minutely pitted surface of basaltic 

 rock, but granite worn smooth is distasteful both to plants 

 and animals. Where the rock is friable, a storm tears off 

 together the weed and the stone to which it has made fast. 

 The nature of the rock also determines the quality of the 

 sand or pebbles on the beach. Where rock is stained by 

 infusions of iron, some molluscs which adhere to it, such as 

 limpets or chitons, assume a dark hue to match the back- 

 ground. 



Shale beds round Sydney are bored by Pholas which can- 

 not face the gritty sandstone. But the sandstone is pierced 

 both by the bivalve Venerupis and the crustacean Sphae- 

 roma. 1 Oups in the sandstone are carved by the urchin 

 Toxocidaris (Plate VII, fig. 11). . 



TEMPERATURE. 



For many years records were kept of the temperature in 

 Sydney Harbour by the Government Astronomer of New 

 South Wales. These readings were taken only once a day 

 at 9 a.m. from a thermometer sunk three feet below the 

 surface at Fort Denison, an island in mid-channel. 2 



The following decennial average is based on Russell's 

 "Monthly Record" for the period of 1881-1890, expressed 

 in Fahrenheit. 



1 Hedley, Rep. Aust. Assoc. Adv. Science, viii, 1901, p. 240. 



2 Russell, H. C, Meteorological Observations made at the Government 

 Observatory. 



