478 MESOZOIC TIME. 



had some connection with its origin. Chalk appears to have been 

 forming over the bottom of the ocean, where the depth does not ex- 

 ceed 15,000 feet, ever since the Cretaceous era, and probably from a 

 period long anterior to this. 



The Flint, as stated on page 471, has been attributed to the siliceous 

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

 soundings of various seas, microscopic siliceous shells of Infusoria 

 (Diatoms or Poly cystines) are as abundant as the Rhizopods in the At- 

 lantic, which favors strongly this opinion. There are microscopic float- 

 ing sponges, that becloud the sea-waters at times, as well as the large 

 siliceous and more common kinds, all of which may have contributed 

 to the result. 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. 53), as is usual in these microscopic organ- 

 isms — gives an opportunity for that slow process of concretion which 

 might result in the flints of the Chalk. And the tendency to aggrega- 

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

 body is undergoing chemical change or decomposition, explains the 

 frequent occurrence of fossils within flints, and the silicification of 

 shells. 



2. American Geography — The Cretaceous beds of New Jersey 

 and of the rest of the border region of the continent, east and south, 

 show, in their structure and position, and in the character of their fos- 

 sils, that they were formed either along a sea-coast or in oif-shore shal- 

 low waters. The limestones of Texas indicate a clearer sea ; while 

 the soft sandy and clayey formations to the north and northwest are 

 evidence that the same sea spread in that direction, but was mostly of 

 diminished depth. In the closing part of the Cretaceous, in the Rocky 

 Mountain region, there was a change permanently from a condition of 

 general submergence under salt water, to one of oscillations between 

 emergence and submergence ; and this condition continued on through 

 a long era in the Eocene Tertiary, if the Coal series, excepting the 

 lowest part, is of that age. 



The outline of the Cretaceous formation over the continent points 

 out approximately the outline of the sea in the Cretaceous period, and 

 the general form of the dry land. This is presented to view in the 

 accompanying map, in which the white part is the dry land of the con- 

 tinent, and the shaded the Cretaceous area, and therefore the sub- 

 merged portion. 



The line of the coast on the east extended from a point in New 

 Jersey, to the southeast of New York City, across to the Delaware 

 River, whose course it followed : this river, therefore, emptied into 

 the Atlantic at Trenton ; and the regions of the Delaware and Chesa- 



