PORT JACKSON SILT BEDS. 175 



strata) free apparently from any deep seated faults but accompanied 

 with numerous joints, shows, that the mechanical force which 

 produced the tilting was gradual, the great thickness of its bed of 

 fresh water origin measuring over 1000 feet deep, now rising only 

 a few hundred feet above the sea level shows that a gradual 

 subsidence has occured of considerable depth. Such a subsidence 

 must have completely changed the appearance of the country, 

 inducing by its great strain on the rocks below, dislocations in the 

 earth's crust, as indicated at the present time by the deep chasms 

 and precipitous gorges of the Hawkesbury River and Blue 

 Mountains, also producing fractures of immense depth which have 

 since been filled with ejections of molten trap. This eruption of 

 igneous rock is especially denoted in Mount Prospect, which is at 

 the northern extremity of the big Prospect dam, and in the belt 

 of basalt extending for many miles along the top of the divide of 

 the Parramatta and George's Rivers. 



In this revolution of the natural features of the country there 

 is reason to infer from an old river drift somewhat similar in its 

 composition to the present gravel beds in the Nepean River near 

 Penrith, disclosed in a deep cutting on the Western line five miles 

 from Penrith, and 500 feet above the river level there, that the 

 same differential movements, so perceptible in the neighbourhood 

 of Liverpool and on the West Coast of Scotland have occurred here, 

 extending through to the valley of the Darling River. 



According to Mr. J. M. Reade, F.G.S., this differential movement 

 is at the present time strikingly exemplified in the change of level 

 taking place on the Baltic Coast. This scientist declares that 

 whilst the Swedish Coast has been steadily rising, the Southern 

 Coast has been as persistently falling. In 134 years the North 

 of Sweden has risen 7 feet, the rate of elevation declining gradually 

 to 1 foot at the JSaze and zero at Bornholm which remains at the 

 same level as in the middle of last century. 



The appearance of Port Jackson at the close of the Triassic 

 period must have somewhat resembled, though on a much more 

 extensive scale, the gorge of the Nepean River near the junction 

 of the Warragamba River, just before the valley widens out into 

 the Camden flats, and the rolling country on each side of them. 



Subsidence and erosion together have produced its present 

 features. The character of the bottom stratum of silt, free 

 apparently from marine mollusks and the presence of only a few 

 whole ones in greyish silt above it, indicate that these silt beds 

 were deposited as lacustrine and estuary format ions. It is probable 

 that the subsidence of Port Jackson and the Parramatta valley 

 began at the close of the Triassic period. Since then climatic 

 influences have assisted in scooping out its channel in the softer 

 shales, wearing it down till it exposed the harder beds of sandstones. 



