CARBONIFEROUS PERIOD. 321 



at what epoch the disturbance took place ; and it may have long 

 preceded the Carboniferous age. More investigation is required to 

 fix the date. 



No example of Carboniferous beds lying unconformably on tilted 

 Subcarboniferous beds has yet been observed in Pennsylvania, Vir- 

 ginia, or New Brunswick. 



In Great Britain, Russia, and the most of Europe, the Carbonife- 

 rous and Subcarboniferous beds, when occurring together, are con- 

 formable. But in central and southern France, as Murchison says, 

 the two are always unconformable. In Bavaria, also, at Hof, the Sub- 

 carboniferous limestones and Devonian follow one another regularly, 

 though inclined together at a large angle; while the coal fields of 

 Bohemia lie in horizontal strata over their tilted edges. 



2. CARBONIFEROUS PERIOD (14). 



Epochs. — 1. Epoch of the Millstone grit ; 2. Epoch of the Coal 

 measures. 



1. EPOCH OF THE MILLSTONE GRIT (14 a). 



I. Rocks : kinds and distribution. 



The Carboniferous period opened with a marked change over the 

 continent. The Subcarboniferous limestones and shales which 

 were formed upon the submerged land became covered with ex- 

 tensive gravel or pebble beds, or deposits of sand ; the beds of that 

 epoch, hardened into a gritty rock, make up the millstone grit and 

 sandstone which underlie the Coal measures. 



Similar conglomerates and sandstones were formed afterwards in 

 the course of the Coal measures ; but this rock is prominent for its 

 extent, and for marking the commencement of the Coal era. 



The Coal period in Britain has in general the same kind of intro- 

 duction : the term Milhtone grit applied to the rock comes to us 

 from England. 



This Millstone grit extends over parts of some of the southern 

 counties of New York, with a thickness of 25 to 60 feet ; and, owing 

 to the regularity of the joints, it stands out in huge blocks, walls, 

 and square structures, that have suggested such names as " Rock 

 City" and "Ruined City" (Cattaraugus and Allegany cos.). It 

 occurs through all the Coal areas of Pennsylvania, both the eastern 

 and western ; it is from 1000 to 1500 feet thick about the centre 

 of the anthracite region, and diminishes rapidly to the westward. 

 It stretches southwestwar.d through Virginia to Alabama. 



22 



