454 



MESOZOIC TIME. 



which end with it, and also for the appearance, during its progress, of 

 the modern types of plants. 



The name Cretaceous is from the Latin creta, chalk. The Chalk 

 of England and Europe is one of the rocks of the period. 



Epochs. — 1 



Later Cretaceous. 



1. American. 

 Epoch of the Earlier Cretaceous ; 2. Epoch of the 



I. Rocks : kinds and distribution. 



The Cretaceous beds occur (1) at intervals along the Atlantic Border 

 south of New York, from New Jersey to South Carolina ; (2) ex- 

 tensively over the States along the Gulf Border, thence bending north- 

 ward along the Mississippi valley, nearly or quite to the mouth of the 

 Ohio, over what was then a great Mississippi bay ; (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, 

 and westward through Dakota, Wyoming, Utah, and Colorado terri- 

 tories ; farther west, along some parts of the valley of the Colorado 

 River, but not over the plateau between the Sierra Nevada and the 

 Wahsatch Range ; (4) along the Pacific Border, in the Coast ranges 

 west of the Sierra Nevada ; (5) in British America, on the Saskatch- 

 ewan and Assiniboine ; also (6) on the Arctic ocean, near the mouth 

 of the Mackenzie, and in North Greenland. On the Atlantic Border, 

 they are unknown north of Cape Cod. 



The formation has its greatest thickness in the Rocky Mountains. 

 The accepted subdivisions for the Rocky Mountain region are, begin- 

 ning below, (1) the Dakota group, which has often a conglomerate bed 

 as its base ; (2) the Colorado group ; (3) the Fox Hills group ; the max- 

 imum thickness of the whole, in the Wahsatch Range, 8,000 or 9,000 

 feet. The beds are marine, but some coal beds are included in the up- 

 per portion. Above No. 3 there is a gradual passage into a group of 

 brackish water strata 4,000 to 5,000 feet thick, including many coal- 

 beds, which is called the Laramie group, or the " Lignitic " formation. 

 It is allied to the Cretaceous in its Dinosaurs, and to the Tertiary in 

 its fossil plants, and is thus intermediate in its life between the Creta- 

 ceous and Tertiary. It is placed beyond in the Tertiary (p. 490), but 

 is as rightly made No. 4 of the Cretaceous, as done by many geologists. 



On the map, p. 144, the Cretaceous areas are indicated by broken 

 lines running obliquely from the right above to the left below : one 

 area crosses New Jersey ; other outcrops on the Atlantic Border are 

 indicated by the lettering Or; an extensive area covers the Gulf 

 States ; and another, the region west of the Mississippi. 



