GEOLOGICAL CONCLUSIONS. 399 
These are examples of the comparatively rapid formation 
of caverns. The waters which run or percolate through them 
must be charged with carbonic acid to accomplish such work, 
and yet they have no source for this ingredient except the atmos- 
phere, animal respiration, and vegetable and animal decom- 
position in the soil. The flutings and stalactitic incrustations 
of a precipice facing the sea must depend on the former alone, 
with the aid perhaps of the spray from the sea brought over 
the reef by storms. 
XIII. OCEANIC TEMPERATURE. 
Facts seem to indicate—though perhaps not sufficient to 
demonstrate—that the Gulf Stream has had, from the Juras- 
sic period in Geological history onward, the same kind of in- 
fluence on the temperature of the North Atlantic Ocean which 
it now has. 
The existence of a coral reef made out of corals of the As- 
trea tribe and others, during part of the Odlitic era (middle 
Jurassic), in England, as far north as the parallel of 52° to 
55° is strong evidence that the isocryme of 68° F., the coral- 
reef boundary, extended then even to that high latitude; for 
species of the Astrea tribe are now confined to coral-reef seas 
(p. 109). This isocryme now reaches along the course of the 
Gulf Stream, to a point just north of the Bermudas, near 
33° N.; and 55° is 22° beyond this. 
There are no marine fossils in any rocks of that period on 
the American side of the Atlantic, so that facts fail for defi- 
nitely locating the western terminus of this odlitic isocryme of 
68° F. But it is highly improbable that the whole ocean 
across, on, or near, the parallel of 55° N., should have had, as the 
mean temperature for the coldest month of the year, one so high 
as 68° F’.; the present average position of the isocryme of 68°F... 
