7O THE HISTORY OF THE NORTH ATLANTIC 



bottom currents of the sea is another cause, and, as we shall 

 soon see, most effective. Still another is the greater cooling 

 and hardening of the crust in the polar regions, and the ten- 

 dency to collapse of the equatorial protuberance from the 

 slackening of the earth's rotation. Besides these, the internal 

 tides of the earth's substance at the times of solstice would 

 exert an oblique pulling force on the crust, which might tend 

 to crack it along diagonal lines. From whichever of these 

 causes, or the combination of the whole, we know that, within 

 the Laurentian time, folded portions of the earth's crust began 

 to rise above the general surface, in broad belts running from 

 north-east to south-west, and from north-west to south-east, 

 where the older mountains of Eastern America and Western 

 Europe now stand, and that the subsidence of the oceanic 

 areas, allowed by this crumpling of the crust, permitted other 

 areas on both sides of the Atlantic to form limited table-lands. 

 This was the commencement of a process repeated again and 

 again in subsequent times, and which began in the middle 

 Laurentian, when for the first time we find beds of quartzite, 

 limestone, and iron ore, and graphite beds, indicating that there 

 was already land and water, and that the sea, and perhaps the 

 land, swarmed with forms of animal and plant life, unknown, 

 for the most part, now. Independently of the questions as to 

 the animal nature of Eozoon, I hold that we know, as certainly 

 as we can know anything inferentially, the existence of these 

 primitive forms of life. If I were to conjecture what were 

 these early forms of plant and animal life, still unknown to us 

 by actual specimens, I would suppose that, just as in the Palaeo- 

 zoic, the acrogens culminated in gigantic and complex forest 

 trees, so in the Laurentian, the algae, the lichens, and the 

 mosses grew to dimensions and assumed complexity of struc- 

 ture unexampled in later times, and that, in the sea, the 

 humbler forms of Protozoa and Sea Mosses were the dominant 

 types, but in gigantic and complex forms. The land of this 



