THE PEN N SYLVAN I AN PERIOD. 551 



widespread west of the Great Plains, and probably underlie the Plains 

 themselves. With rare exceptions, these western beds, largely of 

 limestone and sandstone, represent the coal-less phase of the Penn- 

 sylvanian (Upper Carboniferous) system, a phase which is far more 

 widespread, the whole earth considered, than the coal-bearing phase. 

 There is probably no other continent where the rocks of the period 

 are so generally coal-bearing as in North America; yet even here the 

 coal-less phase of the system is probably more extensive than the 

 coal-bearing. The abundant coal of the west does not belong to this 

 system, but to later ones (Cretaceous and Tertiary). While there- 

 fore there are large areas where coal constitutes a part of the Carbon- 

 iferous system, the formations of this period do not all contain coal, 

 nor are all workable coal deposits referable to this period. In this 



Fig. 245. — Section in southwestern Massachusetts, showing the position and rela- 

 tions of the Carboniferous system. Cw = igneous rock, Carboniferous; Sc (Con- 

 way schist) and Sg (Goshen schist) are Silurian formations; Oh (Hawley schist), 

 Os (Savoy schist) and Och (Chester Amphibolite) , are probably Ordovician, though 

 classed with the Silurian in the Hawley folio. (Emerson, U. S. Geol. Surv.). 



case, as in others yet to be described, the exceptional and relatively 

 local phase has been taken for the type, partly because it was the part 

 first studied, and partly because it is of great industrial importance. 



The Mississippian and Pennsylvanian systems have not always 

 been differentiated west of the Great Plains. With rare exceptions, 

 the formations of this system (or of the two systems together) are 

 such as to indicate that marine conditions prevailed in the west. 

 Locally at least (p. 508), sedimentation was interrupted at the close 

 of the Mississippian period, and the tendency of the rapidly increasing 

 knowledge of the west is toward the view that unconformity between 

 the Lower and Upper Carboniferous is somewhat widespread. 1 Fur- 

 thermore, the formations of the Pennsylvanian period are not in gen- 

 eral so widespread as those of the Mississippian, thus affording evidence 

 as significant as that of unconformity of the geographic changes of the 

 time. 



From the nature of the formations, it is clear that sedimentation 

 in the western part of the continent was effected under conditions 



1 See foot-note, p. 510. 



