§7. CHRONOLOGICAL SYNOPSIS. 201 



lacustrine deposits, containing remains of animals and 

 vegetables of all the existing orders and families, and many 

 genera and species ; and with these are associated numer- 

 ous extinct species and genera of mammalia, reptiles, fishes, 

 mollusks, &c. 



Observatioiis. — The formations of this epoch are assemblages of 

 marine, fluvio-ruarine, and lacustrine strata, either isolated or alter- 

 nating, and occupying extensive but denned areas. These beds con- 

 sist of detritus, accumulated during long periods, in bays, gulfs, deltas, 

 and lakes, and in the depths of the ocean. Intrusions of lava currents, 

 scoriae, and other volcanic products, occur in many of the tertiary for- 

 mations. The zoological character of this period is the prevalence of 

 terrestrial mammalia. 



III. The Secondary Epoch. — An immense series 

 of marine strata, with intercalations of fluvio-marine de- 

 posits, and one extensive delta. The most striking zoolo- 

 gical characters of this epoch are the almost entire absence 

 of warm-blooded animals, and the abundance of terrestrial 

 and aquatic reptiles. The aggregate thickness of the strata 

 is many thousand feet. The following are the principal 

 subdivisions of the formations comprised in this class. 



1. the cretaceous (or Chalk) formation. — Marine 

 strata, with but few interspersions of terrestrial or fluviatile 

 debris. In England, the white limestone, termed Chalk, 

 forms the upper strata ; marls and clays the medial ; and 

 clays, sands, and sandstones, the lower portion of the series. 

 The strata abound in the remains of extinct species of 

 zoophytes, mollusca, echinoderms, fishes, and reptiles. Drifted 

 wood, and some marine plants are, with but few exceptions, 

 the only vegetable exuviae. 



Observations. — The strata comprehended in the term Cretaceous 

 Formation, though differing essentially in their lithological consti- 

 tuents, so far correspond in the nature of their organic remains, as to 

 prove that the whole were formed under the same general conditions ; 

 in other words, that the sea and land, and their inhabitants, underwent 

 no essential change during the vast periods in which these deposits were 



