TUB LOWER AND UPPER SILURIAN AGES. 67 



twenty feet or more in height, its solitary animal at 

 the top like a pillar-saint, though no doubt more ap- 

 propriate and comfortable; and multitudes of delicate 

 and encrusting corals clinging like mosses or lichens 

 to its sides. This creature belongs to the very middle 

 of the Silurian, and must have lived in great depths, 

 undisturbed by swell or breakers, and sheltering vast 

 multitudes of other creatures in its stony colonnades. 

 Lastly, the Silurian corals flourished in latitudes 

 more boreal than their modern representatives. .In 

 both hemispheres as far north as Silurian limestones 

 have been traced, well-developed corals have been 

 found. On the great plateaus sheltered by Laurentian 

 ridges to the north, and exposed to the sun and to the 

 warmer currents of the equatorial regions, they flou- 

 rished most grandly and luxuriantly : but they lived 

 also north of the Laurentian bands in the Arctic Sea 

 basins, though probably in the shallower and more 

 sheltered parts. Undoubtedly the geographical ar- 

 rangements of the Silurian period contributed to this. 

 We have already seen how peculiarly adapted to an 

 exuberant marine life were the submerged continents 

 of the period ; and there was probably little Arctic 

 land producing icebergs to chill the seas. The great 

 Arctic currents, which then as now flowed powerfully 

 toward the equator, must have clung to the deeper 

 parts of the ocean basins, while the return waters from 

 the equator would spread themselves widely over the 

 surface ; so that wherever the Arctic Seas presented 

 areas , little elevated out of the cold water bottom, 



