306 BOTANY AND PALEONTOLOGY 



marks, which generally look like large worms of sandstone, incrusted in a 

 matrix of the same matter. But I was nnable to discover in them any 

 trace of organism, or any general typical form to which they could be 

 referred. Their outline is very irregular; sometimes they appear long, 

 linear, of equal thickness (generally half an inch) in their whole length ; 

 sometimes they are constricted, and apparently cut into pieces of unequal 

 size; sometimes they are thicker, short, and even perfectly round. I 

 suppose that they are pure mechanical concretions, formed by infiltration 

 or percolation of water, charged with carbonate of lime or oxide of iron 

 at the time when the sandstone was yet a soft sandbank. The extra- 

 ordinary horizontal extent of the sandstone bearing these marks is never- 

 theless a fact apparently contradictory to this explanation; for it appears 

 near the top of all the conglomerate hills of the coal-measures of Arkansas, 

 when they are high enough to reach its geological horizon. But the 

 nature of the overlying strata might have influenced the infiltration of 

 foreign substances over a vast area.* 



MOUTH OF SPADRA CREEK. SPADRA COAL.f 



The shales covering this coal bear already, like those of the Horsehead 

 Creek coal-bed, traces of a metamorphism which has hardened them and 

 split them contrary to the plan of stratification. This renders them brittle, 

 and causes under the stroke of the hammer irregular fractures which 

 prevent the preservation of fossil plants. The shales are grayish or black, 

 less micaceous than at Horsehead Creek, and more like those of Male's 

 coal-bank. The few plants determinable in the broken pieces of shale are, 

 Neuropteris tenuifolia, Brgt. ; an abundance of leaves of Lepidodendron 

 and Lepidopliyllum lanceolatum, Brgt. The coal is overlaid by the same 

 brash coal as that of James' Fork and other places, which contains espe- 

 cially in abundance Calamites undulatus, Brgt., and Catamites pachy derma, 

 Brgt. These species, like the former, show the same geological horizon 

 for this coal as for the other beds examined in Arkansas. At some places, 

 near the mouth of Spadra Creek, the coal is three and a half feet thick, 

 including a clay parting of three inches, and about six inches of brashy 

 coal. It is still underlaid by the black, hard fire-clay full of leaves of 

 Stigmaria which has been mentioned before. The same coal crops out 

 above the town of Spadra, on the bank of the Arkansas River, where it is 



* To give an idea of the difficulties attending a botanical exploration at this season of the year, 

 I may mention that, on the 12th of November we ascended the mountain with a strong, cold 

 north wind and snow. 



t See description of this coal by the State Geologist, in the first volume of the Report, page 

 129. * 



