250 



THE GEOLOGICAL HISTORY OF PLANTS. 



other distributions that would give very different results. 

 But this is not an imaginary case. We know that, while 

 the forms and positions of the great continents have been 

 fixed from a very early date, they have experienced many 

 great submergences and re-elevations, and that these have 

 occurred in somewhat regular sequence, as evidenced by 

 the cyclical alternations of organic limestones and earthy 

 sediments in successive geological formations. 



An example bearing on our present subject may serve 

 to illustrate this. In the latter part of the Upper Silu- 

 rian period (the Lower Helderberg age), vast areas of the 

 American continent* were covered with an ocean in 

 which were deposited organic limestones whose fossils 

 show that this great interior sea was pervaded by equa- 

 torial waters bringing food and warmth, while the in- 

 cipient ranges of the Appalachians on the east, and the 

 Cordilleras on the west, and the Laurentian axis on the 

 north, fenced off from it the colder arctic waters. How 

 different must the climate of America and of the region 

 north of it have been in these circumstances from that 

 which prevails at present, or from that which prevailed 

 in certain other periods, when it was open to the incur- 

 sions of the arctic ice-laden currents, bearing loads of fine 

 sediment ! f It was in these circumstances, and in the 

 similar circumstances in which the great Corniferous 

 limestone of the Devonian was deposited — a limestone 

 showing in its rich coral fauna even warmer waters than 

 those of the Lower Helderberg — that the Devonian flora 



* See a memoir and map by Prof. Hall, " Reports of the Regents of 

 New York," 1874-"75. 



f It seems certain that the faunae of the old limestones, like the Tren- 

 ton, Niagara, Lower Helderberg, and Corniferous, belong to warm and 

 sheltered sea areas, and that those rich in graptolites and trilobites, en- 

 closed in muddy sediments, belong to the colder arctic waters. Such 

 arctic faunae are those of the Quebec group and of the Utica shale, and 

 to some extent that of the Hamilton group. 



