A significant physical change marked the close of the Mississippian. This was the withdrawal of sea water from nearly all of the continent, the emergence of the land having been generally sufficient to allow considerable erosion. The fact that the Mississippian and the next, or Pennsylvanian, strata are separated by the most extensive distinct erosion surface in the whole Paleozoic group of rocks is the chief reason for considering those two sets of strata to have formed during separate periods of geologic time. In eastern North America the Mississippian strata vary in thickness from a few hundred feet to a maximum of about 5,000 feet in eastern Pennsylvania. In the West, where the thickness is commonly several thousand feet, limestone greatly predominates. There appears to have been vigorous volcanic activity during the period from northern California to Alaska. Certain profound crustal disturbances marked the close of the period in western Europe, resulting in upturning and folding of rocks during the process of mountain forming from Ireland to Germany, and from Bohemia to southern France. Abundant intrusions and extrusions of molten rocks accompanied the disturbances. We turn next to a consideration of the Pennsylvanian period, which is of very special interest, because within the rocks of that age in North America, Europe, and China occur the greatest known coal deposits. The period opened with almost all of North America dry land undergoing more or less erosion. Early in Pennsylvanian time marine water began to overspread the western side of the continent, especially most of the western two-thirds of the area of the United States, where strata thousands of feet thick piled up. The sea was most widespread before the middle of the period, when the relations of land and water were about as shown by Figure 39. Over the site of the Appalachians and most of the eastern half of the Mississippi Valley area the land either stood near sea level and was often swampy or marshy, or at other times it was a little below sea level, allowing tidewater to overspread the area. Such conditions alternated repeatedly, usually more or less locally, over different parts of the districts in which the great coal mines of the east are located. Under such conditions strata from 1,000 to 8,000 feet thick accumulated. Remarkable physical geography of this kind resulted in the growth and accumulation of vast quantities of vegetable matter which has changed into the world's greatest coal beds. Similar conditions prevailed over parts of Nova Scotia, New Brunswick, and Rhode Island, where strata fully 13,000 feet thick accumulated.