SUMMARY OF MESOZOIC HISTORY 275 



Lower Cretaceous strata occur on the Atlantic and eastern 

 Gulf coasts, where they consist almost entirely of unconsolidated 

 sands and clays of continental origin. The Lower Cretaceous 

 strata in the Texan region are made up chiefly of more or less 

 consolidated sands, sandstones, and chalky limestones of marine 

 origin, with continental deposits at the base. In the western 

 interior regions of both the United States and Canada, the strata 

 rather doubtfully of this age are probably of continental origin. 

 On the Pacific Coast there are great thicknesses of marine Lower 

 Cretaceous strata of rather restricted occurrence. 



Upper Cretaceous deposits of the Atlantic and eastern Gulf 

 regions are mostly sands, clays, marls, and greensands, with some 

 chalky limestones toward the south. These are very largely of 

 marine origin. In Texas and the western interior the Upper Cre- 

 taceous beds are there mostly marine sandstones, shales, and chalky 

 limestones, though some continental deposits (including coal) 

 also occur, especially in the latest Cretaceous. On the Pacific 

 Coast typical marine beds occur. 



Some igneous rocks of Triassic age occur on the Atlantic Coast, 

 while great quantities of Cretaceous igneous rocks occur in the 

 Pacific northwest. 



In general the thickness of the Mesozoic group of rocks is not 

 nearly as great as that of the Paleozoic, but more locally remarkable 

 thicknesses of strata are represented in even single systems, as in 

 the case of the Triassic beds of the Atlantic border (10,000 to 

 15 ; 000 feet thick), or the Lower Cretaceous beds of the Pacific 

 border (fully 26,000 feet thick). 



Physical History 



Relations of Land and Sea. — Throughout the era, except 

 during the Cretaceous, North America was mostly dry land, thus 

 being in marked contrast with the Paleozoic condition of the con- 

 tinent. The eastern half or two-thirds of the continent, except 

 the Atlantic and Gulf borders, was continually dry land, while the 

 western side of the continent was subject to varying marine, 

 estuarine, and lacustrine conditions. The reader should review 

 the paleogeographic maps. 



At the opening of the Mesozoic era, or Triassic period, eastern 

 North America was all dry land; continental (partly lacustrine) 



