ME SO ZOIC ERA.- AGE OF REPTILES. 



351 



to be composed wholly of the remains of low organisms,, 

 chiefly foraminifera (Fig. 304). The flints are seen to be 

 composed of shells of Diatoms and spicules of sponges. 

 Now, as already shown (page 117), this is exactly the 

 composition of deep-sea ooze (globigerina ooze),, except 

 that the silica has been separated and collected in nod- 

 ules. It seems probable, therefore, that chalk is a deep- 

 sea ooze of the Cretaceous times. 



2. Coal. Coal is found, again, in the Cretaceous, both 

 in the United States and elsewhere. But as most of our 

 later coal belongs to a transition period between the Cre- 

 taceous and the Tertiary, we shall put off the discussion 

 of these for the present. 



Life-System ; Plants. 



So great is the change and the advance in plants at 

 this point, that if we were guided by plants alone, we would 

 say that the Cenozoic era commenced with the Cretaceous. 

 Here the present aspect of field and forest seems to begin, 

 for here were introduced for the first time, and in great 

 numbers, dicotyls, or ordinary hard-wood trees. The sud- 

 denness of their appearance, however, is due, in part at 



FIG. 305. FIG. 306. FIG. 307. 



FIGS. 305-307. Cretaceous plants (after Lesquereux): 305. Sassafras araliopsis. 

 306. Salix protesefolia. 307. Fagus polyclada. All reduced. 



