UPPER BARREN MEASURES. PP. 27 



the pebbles are often so numerous and large as to cause it 

 to rival the Great Conglomerate of the Coal Measures. The 

 rock is sometimes a mass of jjebbles from J of an inch to 

 one inch in diameter. This sandstone often descends, and 

 cuts out a portion, sometimes nearly all, of the underlying 

 coal. Immediately under it, come the roof shales of the 

 Waynesburg Coal, which contain the plant impressions 

 which form a considerable portion of those to be described 

 in the following pages. 



The roof shales which yield the plants, are usually from 

 6 to 10 feet thick, of a dark dove color, and quite fine 

 grained. The plants are generally finely preserved. The 

 physical character of these shales is remarkably uniform, 

 and differs but little at widely separated localities, so that 

 it alone is sufficient to decide the horizon of the specimen 

 showing it, especially when containing some of the many 

 plants which it affords. 



The most striking difference shown between the mode of 

 occurrence of the plants in the roof shales of the Waynes- 

 burg Coal and of those found at higher horizons^ is seen in 

 the fact, that in the latter the species are few, while the 

 number of individuals is very great, and these species ex- 

 tend over the entire areas of the coal field. Thus we find 

 a few plants forming the entire flora of localities, when 

 from the immense number of individuals, and from the ex- 

 cellent preservation of the material, we are led to expect to 

 find a great variety. 



At the Waynesburg horizon, on the contrary, while the 

 number of individuals of a species is great, we also find a 

 larger number of species. Again, we find the plants dis- 

 tributed in the most singular manner, they being grouped 

 in colonies, which are confined within very narrow limits ; 

 so tha,t the plants which abound in one opening for coal, 

 will be entirely wantino- in another only a few hundred 

 yards distant, where we find instead of them a collection 

 of species so different, that it might well characterize a 

 different horizon. The same rule holds good at the ex- 

 posures of the bed in other places, but not in so marked a 

 manner as at Cassville. 



