LOWER BARKEN MEASURES. PP. 19 



be considered as representing also the flora of the Lower 

 Productive Measures of West Virginia. Of course, with 

 more extended and careful search, we may expect to find 

 many additional species. The lists are especially valuable, 

 as showing the change which has taken place in the group- 

 ing of the plants since the Conglomerate period. 



Tlie Loicer Barren Measures. 



This series takes its name from the comparatively small 

 amount of workable coal which it contains. It has for its 

 base the Mahoning Sandstone, and extends up to the Pitts- 

 burg Coal bed. Its thickness, in the southern part of the 

 State, is not known, but is perhaps about 700 feet. It is 

 the last of the groups which have their maximum thickness 

 in the South. In the northern portion of the State, its 

 thickness ranges from 550 to 600 feet. 



Its physical character is pretty uniform. The base is 

 comj)osed of a sandstone, (the Mahoning,) which is usually 

 thick and coarse, and quite often conglomeratic. From 

 near the base to the middle portion we find some thin 

 marine limestones. One of them, the highest persistent 

 limestone showing marine fossils, is noteworthy as being 

 the last stratum which gives evidence of the extensive 

 prevalence of marine conditions, and for its great extent 

 and uniform character. Though hardly ever more than 

 two feet thick, it extends over an area in W. Virginia of 

 more than 30,000 square miles, showing everywhere the 

 same lithological character, and containing the same fossils. 



This stratum, the '* Crinoidal Limestone," of the Ohio and 

 Pennsylvania surveys, is of great importance, as a geologi- 

 cal horizon, since it furnishes an easily recognized initial 

 plane. 



Up to this horizon, the incursions of tlie sea were not un- 

 common, as is shown by the marine fossils of the limestones 

 of the underlying groups. Limestones are not uncommon 

 in the succeeding measures above, but they are usually im- 

 pure, and of fresh water origin. There must then, at this 

 point, have been an important change in the physical 

 geography of the coal field. 



