BEACH AT PERNAMBUCO. 243 
Thus it appears that beach-rocks are generally formed by the 
deposition of carbonate of lime; but this is not always the case. 
M. A. Papier has described* a rock, formed on the beach near Bona, 
of which the cementing material is a silicate. At that place he 
found sand and fragments of shells, and also a shingle of quartz 
pebbles, consolidated into a hard rock. The sand-rock effervesced 
slightly with acid, owing to the presence of fragments of shells; but 
the shingle-rock, which was exceedingly hard, showed no signs of 
effervescence whatever, and neither the one nor the other was dis- 
integrated by the strongest acids. 
The beach-rock at Pernambuco has probably not been exposed for 
a long period of time, there being unmistakable signs elsewhere of 
a recent elevation of the coast. Nevertheless it is remarkable that 
it should have resisted the sea for so long a time as it is known to 
have done. ‘The port of Pernambuco owes its existence entirely to 
the presence of this narrow har of sandstone. 
Is it possible that these beaches mark periods of repose in the 
slow vertical movements which have occurred within recent times 
in the northern part of South America, as the successive lines of 
inland cliffs which divide the sloping shingle plains of Patagonia in 
almost unbroken lines for hundreds of miles have been shown by 
Mr. Darwin to mark, on a much grander scale, the periods of repose 
during the slow elevation of the southern part of the continent ? 
Discussion. 
Prof. Duncan said that he thought geologists, as a rule, agreed 
that Prof. Dana’s explanation of the formation of coral-rock was 
correct, though there might be exceptional cases. The amount of 
carbonate of lime introduced into these beaches was remarkable. 
The marine vegetation found at the bases of these rocks seemed to 
show that there was a considerable quantity of carbonate of lime 
in the sea-water. He thought, from the specimens on the table, 
that the author was right In assuming an upheaval. With regard 
to the grooves in the Chalk of the Kentish shore, noticed by the 
author, he had observed that waves meeting made lines of greater 
pressure, which seemed to correspond with these grooves. The 
case of the cementation by a silicate mentioned by the author was 
very interesting. 
Sir Jonn Hawxsuaw had examined the reef, and, from its ap- 
pearance, had conjectured it to be thin, which on boring proved to 
be the case. There are along the Brazilian coast, in some places, 
parallel ridges of sand at much the same distance from the shore as 
the reef. These did not appear to be yet solidified ; but, under 
favourable circumstances, they might be. Behind some of these 
ridges were pools of water, in which are found shrimps which are 
different from those found in the adjoining sea, their antenne 
* Bull. de la Soc. Géol, de France, 3° sér. vol. iii. pp. 46-48, 
