354 Prof. Hartt on the Brazilian Sandstone Reefs. — [June, 
As a result of his careful study of the Pernambuco reef, and of 
many others of similar character, some of which we will men- 
tion farther on, Prof. Hartt has given us the following conclu- 
sions regarding the mode of formation of this class of structures: 
It is very evident that they are not the out-cropping edges of 
beds of sandstone, extending out from the shore, as some have 
supposed, but only narrow strips of stone of slight thickness, 
formed in exactly the same position in which we see them to- 
day, that is, just below the level of high tide. They have resulted 
from the solidification of beach materials, or sea beaches, by car- 
bonate of lime carried into them by the percolating waters. This 
action goes on from the level of mean high tide to a variable, but 
only slight, distance below low tide mark, and has only -a limited 
horizontal extension. By the after encroachment of the sea, 
aided by rivers flowing behind them, these consolidated beaches 
have often been separated from the main shore as distinct reefs ; 
but sometimes this latter action has not taken place, and the har- 
dened layer retains its normal position upon the beach, 
The agencies concerned in the formation of these hardened 
beach deposits are mostly very simple ones, many of which can 
be witnessed by any person visiting the sea-shore. The slope of- 
a sand-beach varies according to the size and character of the 
sand-grains composing it, the exposure of the coast and the 
height of the tides. When a wave strikes upon a beach it rushes 
up the slope in a sheet of foam, carrying with it a quantity of 
sand. As it returns the sand is spread out over the surface ina 
thin layer. In this manner a regular lamination, dipping at a - 
more or less even angle, is produced in the material composing 
the beach. But this regularity is often ‘much disturbed by. 
storms, when the beach may be broken into by the waves and 
much of its material redeposited at quite different angles. 
If the land back of the shore be very low, the beach may form 
a simple narrow ridge, over which the waves completely break at 
high water, carrying and depositing sand on the inner side of the 
beach, where the dip of the laminz will of course be landward. 
Ordinarily, however, a ridge of sand is formed behind the sea- 
beach, above the reach of the tides, being partly due to the 
action of the surf during storms, but mostly to the winds. Suc 
a beach-ridge as this accompanies most of the Brazilian beaches, 
= which latter, on account of the exposed character of the coast, 
=~ are formed of quite clean sand, consisting mainly of rounded 
