400 CORALS AND CORAL ISLANDS. 
through the middle of the two oceans, the Pacific and Atlan- 
tic, is near the parallel of 27° or 28°, or one-half nearer the 
equator than the parallel of 55°. It is difficult to account for 
an oceanic temperature high enough to give England’s seas 
68° FI’. as the average for the coldest winter month, even sup- 
posing the Gulf Stream to have aided; but it is vastly more 
difficult if no such northeastward current existed, and the high 
temperature extended equably so far from the equator. The 
probability is therefore strong that the existence of coral reefs 
in the Odlitic era in England was owing to the extension, by 
the aid of the Gulf stream, of the isocryme of 68° move than 
30° in latitude (and over 3,000 miles in distance) beyond its 
present most extra-tropical position, just outside of the Ber- 
mudas; in other words, that the whole ocean was just enough 
warmer, to allow this oceanic current (part of the great 
water-circulation of the globe) to bear the heat required for 
corals as far north as northern England. 
The present isocryme of 44° F’., as drawn on the chart of 
the world accompanying this volume, has approximately the 
course which that of 68° F. probably had in Odlitic times. It 
should have a little less northing, and the loop to the north 
should lean more to the eastward. The latter would have been 
a consequence of the submerged condition at the time of most 
of the European continent. 
The ocean’s waters seem to have cooled somewhat before 
the next period—the Cretaceous—began, since evidence fails 
of any Cretaceous coral reefs in the British seas; but such 
reefs prevailed then in central and southern Europe, so that 
the amount of cooling in the interval since the Odlitic era, 
had not been large; and as late as the Miocene Tertiary, there 
were reef corals in the seas of Northern Italy, above latitude 
45° N., or that of Montreal, in Canada. 

