Page:Popular Science Monthly Volume 76.djvu/484

480 the land. At present, cycads are almost exclusively tropical, ranging outside only a short distance in eastern Asia. Palms are not quite so delicate, ranging outside the tropics to 34° N. lat. on the west coast of America and to 36° N. on the east coast. But in any case, abundant remains of either point to tropical or subtropical conditions.

Beef-building corals are even more definite in their testimony concerning tropical temperature of the water. They are now found only in the tropics, where the winter temperature does not fall below 68° F. (20° C), and in general between 26° N. and 26° S. But since this temperature zone may be extended by marine currents, coral reefs may sometimes reach beyond 26° N. lat., as in the Bermudas, but more often they fail to reach this geographic limit, as on the west coast of America.

The principal reef-builders, the Madreporidæ and the Astræidæ, are confined to the hottest part of the tropical belt, within 18° of the equator, and where the temperature does not fall below 74° F. (23° 20′ C). Between this line and the isotherm of 68° F. coral reefs occur on both sides of the equator, but they are composed largely of Poritidæ and Milleporidæ.

On the west coast of America the minimum isotherm of 68° F. runs north of the equator, and the Galapagos Islands have no reefs, for the temperature there often falls below 68° F. Reef building corals occur in patches from Panama to Magdalena Bay on. the coast of Lower California, but they do not form any reefs, and are composed almost entirely of Poritidæ.

Fossil deposits of Astrgeidæ, in any age and anywhere, indicate with a reasonable degree of certainty that the sea had a temperature of not less than 74° F., and corals of any of the modern reef-building groups show that the temperature was not less than 68° F.

But the reef building Hexacoralla are not known below the Triassic, and for the Paleozoic era we must use other criteria. From the Cambrian to the upper part of the Carboniferous coral reefs are known, but they are formed by Favositidæ and Tetracoralla, both wholly extinct, so that we can only infer their habits. It is, however, nearly certain that these ancient reef-forming corals lived under the same conditions as the modern groups, and that the temperature of the sea where they lived was tropical.

Absence of coral reefs from any formation does not prove that the temperature of that time was not tropical, for even now coral reefs are lacking in many parts of the tropics, on account of unfavorable conditions other than low temperature. Also the corals of the ancient reefs have often been obliterated by metamorphism, and only massive limestone left.