420 New York State Museum 



of the Great Plains region and most of the land areas of 

 the west and southwest from, and including, Mexico to 

 the Arctic. Shales with limestone and gypsum in the 

 Michigan area likewise indicate a dry climate and times 

 when the bays of the sea were converted into salt lakes. 

 In the Appalachian area the clastic sediments have a 

 maximum thickness of about 4000 feet. Here occur the 

 oldest American coal beds and the Mississippian has 

 therefore sometimes been termed the "false Coal Meas- 

 ures." The deposits of the interior sea are largely lime- 

 stones and have a thickness between 1000 and 2000 feet. 

 In the Cordilleran sea limestones were deposited with 

 maximum thicknesses between 1500 and 2500 feet. In 

 the Acadian province clastic deposits have an aggregate 

 thickness of 4000 feet to 6000 feet, with extensive sheets 

 of igneous rocks at the top of the series. At the close of 

 the Mississippian widespread uplift and withdrawal of 

 the sea resulted in a general unconformity between this 

 and the next period, the Pennsylvanian. Folding oc- 

 curred in several parts of North America, as the southern 

 Appalachian and the Nova Scotia-New Brunswick areas. 

 Though coal accumulation had begun in the Missis- 

 sippian the period of greatest accumulation was in the 

 Pennsylvanian, also known as the Coal Measures. The 

 period is characterized by great coal-making swamps. In 

 certain areas the seas were most oscillatory, local warp- 

 ings resulting in periodic shallow water conditions. In 

 the Appalachian trough and in the central interior sea 

 there are alternations of marine deposits with coal accu- 

 mulations. In the Appalachian trough sediments carried 

 in by the streams brought about intermittent sinking, and 

 at intervals this area was slightly above sea level and 

 sediments were continental. At other times marine wa- 



