GENERAL OBSERVATIONS. 493 



genera of the Ammonite family, Crioceras, Hamites, ScapMtea, Baculites, Turri- 

 lites, first appear; also Hippurites and other genera of Rudistes. 



4. Among Vertebrates. — The Mosasaurus, and a few other Reptiles, besides 

 many genera of Fishes. 



The prospective characteristics allying the Cretaceous with after-time are 

 mentioned beyond, in connection with the General Observations on the Meso- 

 zoic. 



GENERAL OBSERVATIONS ON THE MESOZOIC. 

 I. Time-ratios. 



The estimate of the comparative lengths of the Palaeozoic ages is 

 given on page 386. According to it, the lengths of the Silurian, 

 Devonian, and Carboniferous Ages are approximately as the ratio 

 3 : 1 : 1. The facts in European geology probably lead to the same 

 result ; the doubt arises from the uncertain thickness of the Pri- 

 mordial rocks. 



The Mesozoic formations in America are too incomplete to be 

 used in such a calculation. In the Western Interior region the 

 whole thickness is only 5000 feet ; and on the Atlantic border 

 nothing definite is yet ascertained. The Mesozoic rocks of Europe 

 and Britain afford more satisfactory data. The maximum thickness 

 of the Triassic (in Germany) is about 3400 feet, 1000 of it lime- 

 stone ; of the Jurassic, 5200 feet, 1000 of it limestone ; of the Cre- 

 taceous, 2400 feet, 1200 of it limestone : hence, for the whole Meso- 

 zoic, about 7800 feet for sedimentary beds, and 3200 for limestone. 

 Making the calculation from these data, as on page 387, allowing 5 

 feet of sedimentary beds to correspond to 1 of limestone, the re- 

 sulting number is 23,800 ; whence the time-ratio for the Palaeozoic 

 and Mesozoic is nearly 3J : 1. 



The time-ratio on the above data for the Triassic, Jurassic, and 

 Cretaceous is 7400 : 9200 : 7200, or approximately, for the three 

 periods, a ratio of 1 : \\ : 1. 



Adopting D'Orbigny's conclusions with regard to the thickness 

 of the European Cretaceous (which are far from established), the 

 ratio between the Palaeozoic and Mesozoic is approximately 2:1. 



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 



