CRETACEOUS PERIOD. 481 



seas ; but the coral reefs of central and southern Europe show that a 

 large part of that continent was within the Cretaceous coral seas, or 

 what is called the sub-torrid zone on the map of oceanic temperature. 

 The warming influence of the Gulf Stream was less than in Jurassic 

 times (p. 452). The present position of the winter line of 48° F., if 

 drawn on the Physiographic chart, would probably run near that oc- 

 cupied by the line of 68° F. in the latter part of the Cretaceous period, 

 except that the submergence of much of Europe would have given 

 a very different sweep to the Gulf Stream. 



The occurrence of a group of stones in the white chalk of southern 

 England, the largest of syenyte and weighing forty pounds, appears to 

 indicate, as Mr. Godwin Austin states, that there must have been float- 

 ing ice in the sea at times. 



There is a difference, in the later Cretaceous, between the species 

 of northern and southern Europe, and also between those of the 

 northern and southern United States, as explained on page 478 ; and 

 this difference is probable due to diversity of temperature. There is 

 a wide difference in North America, in the life of the two regions, 

 Texas and the Upper Missouri ; but, as Meek has remarked, this may 

 be largely owing to the difference in the horizon of the beds, and also 

 to that of the clearness or purity of the waters. 



4. GENERAL OBSERVATIONS ON THE MESOZOIC. 



I. Time-ratios. 



An estimate of the comparative lengths of the Paleozoic ages is 

 given on page 381. According to it, the lengths of the Silurian, 

 Devonian and Carboniferous Ages are approximately as the ratio 4 : 

 1:1. The facts in European geology lead to probably the same re- 

 sult ; the doubt arises from the uncertain thickness of the Primordial 

 rocks. 



The thicknesses of the Mesozoic formations lead, in a similar man- 

 ner, to the time-ratio for the Paleozoic and Mesozoic nearly 4 : 1, and 

 for the Triassic, Jurassic, and Cretaceous approximately 1 : 1^ : 1. 



II. Geography. 



Through the Mesozoic, North America was in general dry land ; 

 and on the east it stood a large part of the time above its present 

 level. Rocks were formed on its southeastern and southern border, 

 and over its great Western Interior or Rocky Mountain region. 

 Europe, at the same time, was an archipelago, varying in the extent 

 of its dry lands, with the successive periods and epochs., Rocks were 

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