August,. 1941 The Queensland Naturalist 
135 
are sedimentary formations, both were laid down in the bed 
of a Cretaceous sea, but the circumstances under which each 
one was built up, are so contradistinct, as to render them 
incomparable to the average eye witness. 
The South Downs of England are composed of vast cal- 
careous layers of chalk, containing secondary siliceous deposits 
of chalcedony. At one time, this area was completely covered 
by a deep sea, and its bed was similar in many respects to 
that of the adjacent Atlantic Ocean to-day. 
Sea water contains countless billions of minute protozoa, 
known as formanifera. These are one-celled creatures each 
protected by a tiny calcareous shell of numerous strange and 
beautiful shapes. The animal obtains its food by embracing 
small particles in its semi-liquid pseudopodia, after which the 
material is absorbed to nourish the tiny creature. As time 
goes by the small shells of these foramanifera are continually 
sinking down to the depths of the ocean to form deposits known 
to us as oozes. At great depths they are undisturbed by 
ocean currents, and at some future date a geological move- 
ment of the lithosphere may bring them above the surface 
of the water, to form such land as we find in the South of 
England to-day. At a later date, water containing silica in 
solution, and a percentage of organic matter percolates through 
the soft rocks, and deposits its burden in concentric layers 
around the sides of cavities in the chalk. Eventually these 
cavities became filled, and remain as nodules of flint, to supply 
the material from which our distant ancestors manufactured 
their simple implements of daily use. The birth of industry, 
and the first stairway in the ascent of man. 
The scene changes from the lush green pastures and copses 
of England. The rose gardens and the little brooks fade 
behind the heat shimmers, and sink below the borders of the 
sea. 
The bare, grey Libyan Desert, with its dusty treacherous 
plains, its ragged escarpments, and its white sand dunes, 
stretching to the silent crests of time worn residuals, the 
valleys in the seas of another day. Your footsteps break the 
silence with the tinkling of dead sea shells, and the wind 
moves the restless sand where once, long ago the sea waves 
lapped and sparkled beneath a tropic sun. 
The vast areas in this part of Africa are composed of 
sheets of limestone in various stages of denudation. They 
were for the most part laid down on the bed of a shallow tropic 
sea, and the decomposition of the many exoskeletons of marine 
organisms such as coelenterata, echmoderms, molluscs, etc., 
have been converted throughout the ages into a hard cal- 
careous rock heavily impregnated with the shells from which 
it originated. The old, smoothly contoured sea bed has been 
further weathered by the action of wind and rare torrential 
storms, and as is the case in most horizontally stratified desert 
areas, the elements have cut through the protecting upper 
layers of rock and have levelled off the surrounding country, 
leaving behind low table mountains, residuals or remnants 
capped and protected by a remaining fragment of the firmer 
on their crest. 
