468 MESOZOIC TIME REPTILIAN AGE. 



lined closely from the left to the right, is Tertiary ; and it pro- 

 bably covers Cretaceous throughout. The part of the Rocky 

 Mountain region more openly lined in the same direction has a 

 surface of fresh-water Tertiary; but Cretaceous beds, in many 

 places at least, lie beneath. 



The rocks comprise beds of sand, marl, clay, loosely-aggregated 

 shell limestone and compact limestone ; they include in North 

 America no chalk. 



The sandy layers predominate. They are of various colors, — white, 

 gray, reddish, dark green ; and, though sometimes solid, they are 

 often so loose that they may be rubbed to pieces in the hand, or 

 worked out by a pick and shovel. Layers of potter's clay occur in 

 the series. 



The dark-green sandy variety constitutes extensive layers, and 

 goes by the name of Green-sand; and, as it is valuable for fertilizing 

 purposes and is extensively dug for this object, it is called Marl in 

 New Jersey and elsewhere. This Green-sand owes its peculiarities 

 to a green silicate of iron and potash, which forms the bulk of it, 

 and sometimes even 90 per cent., the rest being ordinary sand 

 mixed with it. There is a trace of phosphate of lime, evidently 

 derived from animal remains, — as animal membranes and shells 

 contain a small percentage of phosphates. It is supposed to owe 

 its value in agriculture to the potash and the phosphates. Fossil 

 shells are very abundant in many of the arenaceous and marly beds, 

 and in some they lie packed together in great numbers, as if the 

 sweepings of a beach, or the accumulations of a growing bed in 

 shallow waters, sometimes cemented together, but generally loose, 

 so as to be easily picked out by the fingers. 



The Cretaceous limestones in Texas are firm and compact, and 

 some beds contain chert distributed through them, as the flint 

 through the Chalk of England. 



The Cretaceous formation has a thickness in New Jersey of 400 

 to 500 feet ; in Alabama, of 500 to 600 ; in Texas, of about 800 ; and 

 in the region of the Upper Missouri, of 2000 to 2500 feet. 



The two epochs, that of the Earlier and that of the Later Creta- 

 ceous, are represented in the Western Interior region (including 

 Texas) ; while on the Atlantic and Gulf borders, in New Jersey, 

 and elsewhere, the beds, exclusive perhaps of the lowest unfossili- 

 ferous layers, belong to the Later Cretaceous. 



The interior limit of the Cretaceous formation (see map, p. 133) follows a 

 line across New Jersey from Staten Island to the head of Delaware Bay; 

 across Delaware to the Chesapeake; across Maryland between Annapolis and 

 Baltimore, southwest into Virginia ; occurs at Elizabeth on Cape Fear River in 



