216 
FLORIDA STATE GEOLOGICAL SURVEY. 
Lithologic Characteristics:—The Key Largo limestone is extremely 
variable in appearance and structure. Like the reef rock of the south¬ 
ern Pacific, described by Dana; 1 it is “solid limestone of coral origin; 
* * ' * in some parts it is a coral conglomerate; * * * over much 
larger areas a fine white limestone; * * * it is often free from any 
proofs of an organic origin. It breaks with a conchoidal fracture, a 
splintery surface, and rings under the hammer. * * * Other por¬ 
tions are made of standing corals with the intervals filled in by reef 
debris, and the whole cemented solid.” 
Since solidification of the rock, there has been much solution and 
redeposition of calcareous matter. In places the rock contains heads 
of coral replaced by clear, rather coarsely crystalline calcite, showing 
quite plainly the structure of the original head. In places the rock is 
a typical breccia composed of angular and cherty fragments in a limy 
cement, this cement and many of the fragments being often bright red. 
These breccias evidently represent loose material that has fallen into 
solution cavities, and been recemented, the red color being due to iron 
oxide. The rock is harder within a few feet of its surface than 
below. This is true, not only of the ledges which are bare, but of 
those below sea level. Besides this general surficial hardening, the 
rock shows in many places on unweathered surfaces a thin layer, 
usually less than two inches thick, and never, so far as seen, one foot 
thick, that is darker than the rock below and is finely banded. In this 
layer all traces of the corals and shells visible in the mass of the rock 
disappear. The crust follows slight inequalities of surface and is 
perhaps due to a deposition of limy ooze in pools or hollows which 
began after the elevation of the reef and may be still in progress. 
Some other features of the rock were brought out by the railway 
work along the keys. Measured in terms of a day’s work with a hand 
drill, the rock is twice as hard as the Key West oolite. Yet, while 
hard, it is so honeycombed with solution passages that many borrow 
pits instead of yielding more loose rock than the cubic contents of the 
pit actually yielded less. 
Thickness:—The only information available regarding possible 
variations in thickness of the Key Largo limestone is that of well re¬ 
cords. Its presence below Jackson Square in the city of Key West 
was not shown by the samples of borings described by E. O. Hovey, 2 
but at Key Vaca a well has shown reef rock fully 100 feet thick; at 
Indian Key over 130 feet of reef rock was indicated by a record 
1 Dana, J. D., On Coral Reefs and Islands, Am. Journ. Sci. (2) XIV, 1852, 
p. 76. 
2 Hovey, E. O., Notes on the Artesian Well Sunk at Key West, Florida, in 
1895. Mus. Comp. Zool. Bull. XXXVIII, 1896, pp. 65-91. 
