249 J. C. HAWKSHAW ON THE CONSOLIDATED 
moreover, that at two periods, separated by the time required to 
change the level of the coast by the amount recorded in the vertical 
distance between the layers of rock, the conditions required to pro- 
duce the consolidated beach should have obtained in the same place. 
The cementing material of the Pernambuco rock is carbonate of 
lime. ‘The rock is very hard, and when freshly broken it has a 
vitreous look. This is in a measure owing to the grains of quartz 
which with an occasional grain of magnetic iron ore make up the 
whole of the residue after the carbonate of lime has been removed 
by acid, being scarcely rounded, but having the original vitreous 
surface which they had when first liberated by the decomposition of 
the felspar in the gneiss from which they were derived. 
On many parts of the coast of Brazil long ridges of sand occur, 
separated from the land by lagoons. The percolation of land-water 
charged with carbonic acid derived from the decayed vegetable 
matter in these lagoons through the sand ridges will account for 
the formation of the beach-rock, the water taking up and again 
depositing the carbonate of lime of the shells imbedded in the sand. 
The flood-level of the lagoon-water would determine the level of the 
upper surface of the beach-rock; and that of the lower surface 
would be determined by the cessation of the consolidating action at 
the level at which the sand was saturated by sea-water—that is, 
almost low-water level. Thus the regularity of the form of these 
reefs may be explained. 
Consolidated beaches, however, occur in localities where there is 
little or no vegetation or land-water, as on the shore of the Red 
Sea, where I have seen beach-rock enclosing recent shells. So also 
on the Great Barrier reef, on the north-east coast of Australia, con- 
solidated beaches are frequently met with. The sand is there often 
wholly calcareous, and when consolidated forms a very tough rock, 
which has been described by the late Prof. Jukes in his account of 
the voyage of the ‘Fly’*. He attributes the formation of the rock 
on the coral reefs to the action of rain-water dissolving the carbo- 
nate of lime in the upper layer of coral sand and redepositing it 
lower down, the deposition ceasing at the level where the sand 
became saturated with sea-water. Prof. J. D. Dana, who describes 
the consolidated beaches which occur on many coral islands in the 
South Seas, attributes + the consolidation to the alternate wetting 
and drying of the sand by the rise and fall of the tide, the carbonate 
of lime being taken up by the sea-water and redeposited as the 
water evaporates. On some islands he noticed pebbles of basalt 
on the shore, each of which was coated with a white layer of carbo- 
nate of lime. Mr. Darwin saw somewhat similar deposits on the 
Island of Ascensiont. Prof. Dana also describes some drift-sand 
rocks on coral islands where hills of blown sand have been more or 
less consolidated by the agency of infiltrating water, ‘fresh or 
salt.” 
* ‘Voyage of H.M.S. Fly,’ vol. i. p. 128. 
tT ‘Corals and Ooral Islands,’ p, 152. + ‘Voyage of the Beagle,’ p. 588. 
