14 FLORIDA KEEFS. 
lying each other in such a manner as to present the same irregularity which 
is found in all drift stratification. Layers upon layers are seen resting 
unconformably, dipping in different directions so as to present all the modi- 
fications which may be observed in torrential stratification, each layer fol- 
lowing, with more or less regularity, the course of the flood under which it 
has been accumidated. 
This dip varies for several degrees, but rarely exceeds 7 to 8 degrees 
upon any extensive surface where the sea has had full jjlay. In enclosed 
bays or narrow channels, however, the materials accumulate at much steeper 
angles. In sheltered places, slopes of coral sand are formed at angles of 
20 or even 30 degrees. At the Tortugas, Lieutenant Wright pointed out to 
us accumulations of coral sand sloping at an angle of more than 33 degrees, 
exceeding decidedly, therefore, the steepest slopes of silicious sand-banks. 
This circumstance corroborates the assertion of Mr. Darwin, that coral sand 
possesses an adhesive property which enables it to accumulate at higher 
angles than other loose materials, owing, jjerhaps, to the viscosity of the 
animal matter still pervading the coral fragments. 
By a process not yet fully understood, but to which we shall return here- 
after, these loose collections are gradually cemented into solid rock, 
presenting the most diversified appearance, according to the substances 
of which it is composed. Then we find a coarse breccia, consisting of 
larger fragments of corals and shells, enclosing sometimes coral boulders; 
and this is the sort of rock which generally overlies the immediate surface 
of that portion of the keys which has been formed by the progress of the 
reef, growing in situ. Such rock was seen among the foundations of 
the new lighthouse at Sand Key, where 'the large boulders are very numer- 
ous, and seem almost as fresh as if they had been lying on the spot but 
for a few years. It may be, indeed, that during the hurricane of 1846, the 
whole cap of the reef was renewed at that spot. 
Similar rock, but of more breccia-form character, containing remains of 
shells in greater quantity than I have seen elsewhere in the coral rock, was 
brought up from excavations made in twelve feet water, when the founda- 
tions of Fort Taylor were laid in Key West. The same coarse-grained rock 
is observed, at low water, in many places where the tidal action has Avorn 
away parts of the primitive rock of the keys. Above these materials are 
generally found layers of what seems, at first sight, a coarse-grained oolite, 
but upon closer examination it is seen to consist, not of grains of limestone 
