50 The Aificrican Geologist. January, i898 
even invading the long established dynasties of the old world. 
A brief account of the main characteristics of such an nn- 
portant member of the Carboniferous series can hardly fail 
to be of some interest to geologists and others who desire 
to learn more of this celebrated coal bed and hence it has 
been chosen as my theme. 
Age. The stratigraphical position of the Pittsburg coal 
bed is at the base of the Monongahela River series of Rogers. 
The thickness of this series varies from 250 to 400 feet in 
different portions of the Appalachian field. It also includes 
four other coal beds interstratified with sandstones, limestones 
and shales, but none of these coals have much economic im- 
portance since alj are thin and impure except over quite 
limited areas, so that the Pittsburg bed may be regarded as 
the last of the great coal-making epochs of Carboniferous 
time. 
The lower and middle Carboniferous had passed; the 
animals and most of the plants that characterize them had 
vanished; the great Lepidodendra, Sigillarise, and Calamites 
of the former floras had been succeeded by dwarfed and puny 
species of their tribe, while the tree ferns alone of all the larger 
plants appear to have flourished and attained considerable 
size. The evening of the Carboniferous day was well ad- 
vanced, since marine conditions in the Appalachian field had 
terminated and brackish or fresh water conditions had arisen 
which continued to the close of the Permian. At the end 
of this fetter epoch 1,500 feet of sediments had accumulated 
above the Pittsburg coal -|- the thickness eroded since the 
close of the Palaeozoic, which latter most probably represents 
a much greater thickness of rocks than the 1,500 feet re- 
maining. 
Prof. Fontaine and myself have shown (Report PP. 2nd 
Geological Survey of Pennsylvania) that beginning with the 
horizon of the Waynesburg coal at say about 350 feet above 
the Pittsburg bed, the rocks contain a well defined Permian 
flora, of types common alike to the Permian of Europe and 
to the well recognized Permian beds of Texas (Bulletin G. S. 
A. Vol. 3, pp. 217-218, 1892). Just where in the series this 
flora was introduced we do not yet know because no sys- 
tematic collections of fossil plants have been made between the 
