174 Coral Reefs and the Glacial Period. [April, 



only the requisite slow subsidence occurred in the northern 

 equatorial region. And the absence of a great coral reef 

 development, in the shape of atolls, north of the equator, 

 points, therefore, either to the want of the requisite slowly 

 subsiding area, or to the advent of a cold period subsequent 

 to that occurring over the southern hemisphere, and, there- 

 fore, checking the coral growth. 



3. What now would probably be the condition of things, 

 supposing the extreme glacial climate occurred simul- 

 taneously in both hemispheres ? Agassiz says,* " Let me 

 state that I have not noticed anything to confirm the idea 

 that the glaciers of the northern hemisphere have alternated 

 with those of the southern hemisphere in their greatest 

 extension, as is assumed by those who connect with the 

 precession of the equinoxes the difference of temperature 

 required for the change. The abrasions of the rocks seemed 

 to me neither more nor less fresh in one hemisphere than 

 in the other. . . ." 



Undoubedly, the extreme glaciation in both hemispheres 

 is the most recent of geological changes ; both north and 

 south of the equator it is of younger date than the late 

 Tertiary deposits. Since, however, a Miocene, or Pliocene, 

 fauna and flora may not be of the same age precisely 

 in both hemispheres, time being required for the slow pro- 

 gress of new animals and plants into far latitudes, it follows 

 that the glaciation, though affecting rocks of these ages, 

 and therefore posterior to them, may not be of equal age 

 both in the north and south. Granted, however, that the 

 period of glaciation was approximately the same in both 

 hemispheres, does it not follow that tropical life would be 

 hard put to for a place of abode ? 



In this case the two isotherms of 68° would be made to ap- 

 proach each other from both hemispheres, and the equatorial 

 belt inhabitable by reef-builders very much narrowed ; in 

 fact, it is conceivable that during such a simultaneous 

 maximum of cold in both hemispheres the combined effect 

 on equatorial heat would be such as to squeeze out, as it 

 were, some forms of tropical life, and confine others to very 

 narrow bounds. On the present supposition, therefore, if 

 the climate ameliorated slowly from the time of extreme 

 cold north and south, to the present day, we might expect 

 to find the thickest reefs or the greatest number of atolls close 

 about the equator, always provided that there were areas of 

 subsidence sufficient to allow of the free growth of atolls. 



* See his Report of South American Expedition, as given in " Nature," 

 Aug., 1872, p. 272. 



