CRETACEOUS PERIOD. 473 



flint-nodules, and the matrix is left in a condition of pure carbonate 

 of lime.* 



Extent of Chalk Seas of Cretaceous Times in Europe.— Chalk of 

 nearly homogeneous aspect prevails from the north of Ireland through 

 Middle Europe to the Crimean and Caucasus,! a distance of 1,140 

 miles ; and, in the other direction, from the south of Sweden to the 

 south of Bordeaux, a distance of 840 miles (Lyell). It is evident, 

 therefore, that at that time a deep sea occupied a large portion of 

 Central Europe. The white chalk of England and France is about 

 1,000 feet thick. When we remember the mode in which it has 

 been formed, this thickness indicates an almost inconceivable lapse of 

 time. 



Cretaceous Coal. — Coal is again found in large quantities in rocks 

 of this period in the United States. The mode of occurrence is simi- 

 lar to that found in rocks of other periods; but as most of this 

 coal is found in the Laramie, and as this is a transition group to 

 the Tertiary, we shall put off the discussion to the end of the Creta- 

 ceous. 



Subdivisions of the Cretaceous. — In the localities where the Ameri- 

 can Cretaceous was first well studied, viz., in Xew Jersey and in the 

 western Plains and Plateau region, the lower part of the series was 

 wanting, and hence a great gap was supposed to exist here in the 

 American geological record. But recently the Lower Cretaceous has 

 been found in many widely-separated regions and by different observ- 

 ers, viz., in California, by Whitney (Shasta group) ; in Texas, by Hill 

 (Comanche group) ; \ in Canada, by Dawson (Kootani group) ; and on 

 the Atlantic border, by McGee (Potomac group).* The Comanche 

 group of Hill is probably the most complete, and we therefore adopt 

 this name. It is probable that the lowest of Hill's Comanche group 

 viz., Trinity beds, may be Uppermost Jurassic. 



The Cretaceous series may be conveniently divided into Upper, 

 Middle, and Lower. The subdivisions of these are of course local. In 

 Europe the Tertiary is nearly everywhere unconformable on the Cre- 

 taceous, showing a gap at this horizon ; in America this gap is com- 

 pletely filled in the West by a transition group called the Laramie. 

 The relation between the main divisions of the American Cretaceous on 

 the interior plains and Atlantic border, and of both with the European 

 Cretaceous, as shown in the following table : 



* Wallace thinks that chalk is a coral mud formed in warm seas full of foramimferal 

 life (Island Life, p. 84). 



f Favre, Archives des Sciences, vol. xxxvii, p. 118 et seq. 



% Marcou had long ago (1853) discovered Neocomian in New Mexico, but the discovery 

 was not recognized by American geologists. 



# This is probably a transition to Jurassic. 



