THE PENNSYLVANIAN PERIOD. 541 



The formation is usually so firmly indurated that where it has been 

 tilted its outcropping edge develops ridges. 



The Millstone grit, usually a coarse, conglomeratic red sandstone, 

 occurs in Nova Scotia, where it contains occasional lenses of coal and 

 shale. 



In the western part of America, the Millstone grit formation has 

 not been recognized. While deposits representing the epoch probably 

 occur, they have not commonly been differentiated from the beds 

 below and above. Locally, however, there are conglomeratic beds 

 which may prove to be the time-equivalent of the Millstone grit 

 of the east. If the Carboniferous (corresponding to Pennsylvanian) of 

 the west is not more generally unconformable on the Lower Car- 

 boniferous than present data indicate, the two systems here afford 

 another illustration of the general fact that different regions on the 

 same continent do not always undergo the same changes, or receive 

 similar deposits at the same time. So true is this, that a shallow- 

 water formation, like the Pottsville conglomerate, in the eastern part of 

 the continent, would afford no sufficient warrant for supposing a like 

 formation to have been made in the western. The deposits of the 

 Pennsylvanian (Carboniferous) period in the eastern and western parts 

 of the continent also show that a classification which fits one region 

 may have little applicability to another, so far as details are concerned. 



The Coal Measures. 



Above the Pottsville conglomerate and its equivalents in the cen- 

 tral and eastern parts of the continent lie the formations known collec- 

 tively as the Coal Measures, a series of beds somewhat unlike any 

 heretofore considered. They consist of a great succession of alterna- 

 ting beds of shale, sandstone, conglomerate, limestone, coal, and iron 

 ore. While the succession differs greatly in different regions, shale 

 and sandstone perhaps recur more frequently than any other mem- 

 bers of the series, and in thicker beds. Limestone has a more limited 

 development, and conglomerate is relatively unimportant. The coal 

 and some of the iron ore are in layers interstratified with the clastic 

 beds, and are to be looked upon as strata of rock, the same as the asso- 

 ciated sandstones and shale. Important as the coal and iron ore are 

 from an economic point of view, they make up but a small part of 



