340 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1914. 
ments, does not exceed 64 meters for true corals and 120 meters for 
calcareous alge of the group of millipores. On agcount of these very 
close restrictions the coral reefs of to-day occupy a much localized 
zone in the vicinity of the equator; and the fact that in the oldest 
geologic periods corals extended into the polar regions is of great 
importance in the history of ancient climates. 
These corals present opportunity for another curious observation. 
Darwin, after some very hasty observations, advanced a theory 
accepted by Dana, according to which the construction of coral reefs 
demonstrated a gradual sinking of corresponding regions. According 
to the theory of Murray, now admitted, the coral reefs simply mark 
out submarine volcanic cones, such as exist in great number in the 
Pacific. Some such cones have been recognized by #ceanographers. 
The Nero pointed out 20 of them on a cable route between Japan and 
Hawai, one rising to within 150 meters of the surface, others reaching 
from a depth of 9,000 meters up to 1,200 meters below sea level, etc. 
It is on these cones that organic sediments at first accumulated (and 
that since what may be a very ancient geologic epoch), down to the 
day when coral organisms from the deep, such as the Lithothamnium, 
begin to be established there and approach the surface at the rate of 
50 meters ina thousand years. In fact, and rather paradoxically, the 
corals often play only an accessory réle in the construction of so-called 
coralline reefs, certain of which present scarcely anything except calca- 
reous alow. Then intervene the remains of these calcareous elements 
and the accumulations of various organisms which become established 
on the reef. 
We come finally to sediments of the deeper seas. These are com- 
posed chiefly of organisms which are divided into two principal 
categories—(a) calcareous deposits with globigerina or with ptero- 
pods; _(6) siliceous deposits with radiolarians and diatoms. 
The influence of temperature on the distribution of these organisms 
is very marked. In warm waters the calcareous organisms dominate; 
in cold waters only the siliceous organisms exist, and it thus appears 
how both of them procure the elements of their substance. 
As for lime, which is in fairly appreciable amounts in sea water, 
there are no traces of carbonate concerned, but the sulphates are 
much more developed. Those are transformed by carbonate of 
ammonia coming from the organisms, and the carbonate of lime is 
then secreted. As the decomposition of nitrogenous matter is 
especially rapid in warm waters, these reactions are there favored. 
The calcareous secretion, abundant in regions of uniformly high 
temperature, diminishes in temperate regions where its maximum 
takes place in summer, and nearly disappears in the polar zones. 
Thus the calcareous organisms, the globigerina, play a réle entirely 
dominant. Nearly all the sea bottom contains at least 10 per cent 
