4 8o THE POPULAR SCIENCE MONTHLY 



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 con- 

 cerning 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 Madreporida? and the Astraeidse, 

 are confined to the hottest part of the tropical belt, 1 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 

 Poritida? and Milleporidas. 



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 Poritida?. 



Fossil deposits of Astrgeida?, 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 Favositidse 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 condi- 

 tions other than low temperature. Also the corals of the ancient reefs 

 have often been obliterated by metamorphism, and only massive lime- 

 stone left. 



1 J. D. Dana, "Corals and Coral Islands," 3d ed. (1890), pp. 108-114. 



