314 
Geology of Sydney. 
annually brought down to the sea by that river. But 
we must also add the amount of sand, gravel, and 
stones pushed along its bed. This may be roughly 
estimated and allowed for. These are some of the 
results : — 
“ The amount of solid matter discharged every 
year by that great river, the Mississipi, if piled up on 
a single square mile of the bed of the sea — say, in the 
Gulf of Mexico, where that river discharges itself — 
Fig. S3. — Diagram (after Dana) showing how the contour of hills 
depends on the unequal weathering of harder and softer rocks 
by denudation. 
would make a great square-shaped pile, 268 feet high. 
But the Gulf Stream, sweeping through this gulf, 
carries the materials many and many a mile away; so 
that in course of time it gradually sinks, and spreads 
itself as a fine film or layer over part of the great 
Atlantic Ocean. The mud brought down by the great 
river Amazon spreads so far into the Atlantic Ocean 
as to discolour the water even at a distance of three 
hundred miles. The Ganges and the Brahmapootra, 
flowing into the Bay of Bengal, discharge every 
