24 THE OCEAN RIVER 



as the sediments formed, crabs and fishes, the coral reefs and 

 the tubes of worms, all manner of hard skeletons of animals, 

 became buried in the slowly hardening muds, and today we 

 see them as fossils. Each successive era of submergence and 

 sedimentation had, as the evolution of sea creatures took 

 place, its own types of marine life. Thus we are able to place 

 together in the geological scale of time, as far back as hun- 

 dreds of millions of years, the corresponding rocks of such dis- 

 tant places as the Appalachian Mountains and the Himalayas, 

 both of which at one time were under the waters of a vastly 

 extended Atlantic Ocean. 



Written in the sediments which have become today's lime- 

 stones and sandstones — and even in the quartzites, marbles, 

 and schists to which these have been converted by the tremen- 

 dous pressure of overlying rock — in these alone can we find 

 the recorded history of the River bed. Earlier periods of moun- 

 tain uplift and of continental flooding are probably recorded 

 in rocks that have not yet become available to us. We can only 

 surmise that the Atlantic Ocean, on five separate occasions 

 during a period between one and two billion years ago, spread 

 itself across Europe and Asia and across a great part of north- 

 ern America, and that it receded during the intervening times 

 of mountain building to become a much more restricted ocean 

 than it is today. 



Somewhere before 500 million years ago we can see the story 

 in greater detail. Another great succession of wrinkles in the 

 granite crust, known as the Pre-Cambrian revolution, took 

 place, causing the continents to rise up out of the sea and the 

 oceans to withdraw from their margins. The Ocean River may 

 have pursued a shorter course in its circular passage than it 

 does today, since some scientists say that most of the conti- 

 nental shelf of eastern North America, the West Indies, and 

 the Gulf of Mexico became part of a continuous land mass, 

 which united North and South America into a single conti- 



