312 CORALS AND CORAL LSLANDS. 



period in Geological history onward, the same kind of influence 

 on the temperature of the North Atlantic Ocean which it now 

 has. 



The existence of a coral reef made out of corals of the As- 

 trsea tribe and others, during part of the Oolitic era (middle 

 Jurassic), in England, as far north as the parallel of 52° to 55° 

 is strong evidence that the isocryme of 68° F., the coral-reef 

 boundary, extended then even to that high latitude ; for species 

 of the Astrsea tribe are now confined to coral-reef seas (p. 84). 

 This isocryme now reaches along the course of the Gulf Stream 

 to a point just north of the Bermudas, near 33° N. ; and 55° 

 is 22° beyond this. 



There are no marine fossils in any rocks of that period on 

 the American side of the Atlantic, so that facts fail for defi- 

 nitely locating the western terminus of this oolitic isocryme of 

 68° F. But it is highly improbable that the whole ocean 

 across, on, or near the parallel of 55° N., should have had, as 

 the mean temperature for the coldest month of the year, one so 

 high as 68° F.; the present average position of the isocryme of 

 68° F., through the middle of the two oceans, the Pacific and 

 Atlantic, 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° F. 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 north-eastward 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 Oolitic era in England was owing to the extension, by 

 the aid of the Gulf Stream, of the isocryme of 6.8° more 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 



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