CRETACEOUS PERIOD. 543 



not represented at all, or very feebly; but there is a very 

 extensive development of rocks of Upper Cretaceous age in 

 the United States. According to Dana, " the Cretaceous 

 Eocks occur i. At intervals along the Atlantic border, south 

 of New York, from New Jersey to South Carolina ; 2. Exten- 

 sively over the States along the Gulf 'border ; and 3. Through 

 a large part of the Western interior region, over the slopes of 

 the Rocky Mountains, from Texas northward, to the head- 

 waters of the Missouri on the east of the summit of the chain, 

 and far into the Colorado region on the west. Still farther 

 north-west in British America, they appear on the Saskat- 

 chewan and Assiniboine, and also on the Arctic Sea, near the 

 mouth of the Mackenzie." The rocks of these areas consist 

 chiefly of sands, marls, clays, and limestones ; but it is to be 

 remembered that there is no white Chalk. Green sands are 

 often present, as in New Jersey, where they are called " marls," 

 and are largely worked for agricultural purposes, their fertilis- 

 ing properties being due to the presence of a small percentage 

 of phosphate of lime. 



LIFE OF THE CRETACEOUS PERIOD. As regards the vegeta- 

 tion of the Cretaceous period, the plants of the Inferior divi- 

 sion of the series agree with those of the antecedent Jurassic 

 period in consisting chiefly of Ferns, Cycads, and Conifers. 

 In the Upper Cretaceous Rocks, on the other hand, we find a 

 vegetation composed largely of Angiospermous Exogens, many 

 of which belong to existing genera. 



The Protozoa are very largely represented in the Cretaceous 

 period by Foraminifera and Sponges. The microscopic shells 

 of the former are often excessively abundant ; and the white 

 chalk is to a large extent composed of the exuvia of these 

 minute organisms. Amongst the more important genera may 

 be mentioned Texttdaria, Globigerina, Rotalia, Lituola, Nodo- 

 saria, Flabellina, Cuneolina, Cristellaria, Bulimina, Dentalina, 

 &c. Sponges are very numerous, especially in the Upper 

 Greensand and White Chalk. The most important genera are 

 Siphonia, Ventriculites, Manon, Choanites, Cliona, Scyphia, 

 Chenendopora, Guettardia, and Polypothecia ; but many other 

 forms might be mentioned. 



The Ccelenterates are represented by Corals, belonging 

 mainly to the genera Trochocyathus, Cyclocyathus, Trochosmilia, 

 Parasmilia, Cyathina, Micrabacia, Stephanophyllia, &c. In the 

 Upper Greensand, also, occurs the little Holocystis elegans, long 

 believed to be the last of the Rugose Corals. 



The Echinoderms are exceedingly abundant in the Creta- 

 ceous rocks, but belong mainly to the Echinoids. Crinoids, 



