488 MESOZOIC TIME — REPTILIAN AGE. 



laris, Nucleolites crucifer. The Gryphsea vesicularis is supposed by some to occur 

 also in the Upper Green-sand and the Lower or Gray Chalk, but the form found 

 in these lower portions is regarded by other authorities as a distinct species. 



Genera common to the Later Cretaceous of America and the Upper or White 

 Chalk of Europe : — Mosasaurus, Saurocephalus, Callianassa, Pleurotoma, Fascio- 

 laria, Cypreea, Pidvinites, Cassidulits. There are also in the American Later 

 Cretaceous the three genera Busycon, Pseudobuccinum, and Xylophaga, which 

 have not yet been found as low as the Cretaceous in Europe. 



III. General Observations. 



1. Origin of the Chalk and Flint. — From the absence of vegetable 

 remains and earthy ingredients, the abundance of sponges, and the 

 general character of the fossils, it is supposed that the Chalk was 

 formed at a distance of some miles from shore, where the water 

 was at least 200 or 300 feet deep. The abundance of Rhizopod 

 shells, as already stated, suggests that these were the main mate- 

 rial ; and the recent observation that the lead in deep-sea sound- 

 ings on the north Atlantic has brought up sand composed almost 

 wholly of minute Rhizopods, as published by Bailey, sustains 

 the conclusion. These shells are like grains of sand in size, and 

 are, therefore, ready for consolidation into a compact rock, need- 

 ing no previous trituration by way of preparation ; and thus they 

 are especially fitted for making deep-water limestones. Corals 

 require the help of the waves to reduce them to grains before a 

 rock of compact texture can result. Moreover, the softness or 

 imperfect aggregation of Chalk is probably due to this origin, and 

 particularly to the fact that each grain is a cellular shell, or collec- 

 tion of air-cells, instead of solid. The coral reefs of the Pacific do 

 not under ordinary circumstances give rise to chalk. The only 

 chalk known in coral regions is on Oahu, at the foot of an extinct 

 volcanic cone ; and there it is probable that warm waters had some 

 connection with its origin. 



The Flint, as stated on page 481, has been attributed to the sili- 

 ceous Infusoria of the same waters and the spicula of Sponges. In 

 the soundings from the Sea of Kamtchatka, Bailey found micro- 

 scopic siliceous shells of Infusoria (Diatoms) as abundant as the 

 Rhizopods in the Atlantic, which favors strongly this opinion. 

 The minute portion of silica which the alkaline waters of the ocean 

 can dissolve — especially when the silica is in what is called the 

 soluble state (p. 55), as is usual in these microscopic organisms — 

 gives an opportunity for that slow process of concretion which might 

 result in the flints of the Chalk. And the tendency to aggregation 

 around some foreign body as a nucleus, especially when such a 



