OF ARKANSAS. 131 



mation in which it occurs is comparatively level, undisturbed, and bear- 

 ing little evidence of metamorphism or change by internal heat; while the 

 coals of similar composition in Pennsylvania occur, as we are informed 

 by Hayes and Rogers, only in coal fields and isolated patches, in the most 

 disturbed portions of the Appalachian chain, and are associated with some 

 of the boldest flexures and greatest dislocations of the whole coal region 

 of that State. The nearest rock of undoubted igneous origin to this coal, 

 at present known to me in Arkansas, is situated in Hot Spring county,* 

 some sixty miles, in a direct line, south of the Spadra ; yet, here we have 

 a coal, possessing all the chemical properties of the semi-anthracites, that 

 are usually found in the midst of the most striking evidence of decided 

 igneous action. The inference which I draw from these facts, is that, though 

 granite and other hypogene (nether-born) rocks do not actually reach the 

 surface in Johnson county, as far as at present known, they must be near 

 enough the surface to have exerted an igneous action, sufficient to have 

 permeated the strata, now found on Spadra creek, with heated vapors or 

 gases, that have expelled the greater portion of the gaseous matter ; or 

 else this coal has been subject to some extraordinary chemical agency, by 

 which carburetted hydrogen has been removed. It is hardly possible that 

 the Spadra coal can owe its present composition to any difference in the 

 vegetation from which it was originally produced ; since it would be, in that 

 case, a strange exception to anything previously observed in the bitumi- 

 nous coal fields in any of the States west of the Alleghany mountains. 

 But the peculiar fissured structure of the Spadra coal favors the idea, 

 that the volatile matter has been expelled by a process more rapid than 

 can be attributed to slow chemical changes, unaided by an elevation of 

 temperature ; since the escape of the volatile matter by heat causes an 

 expansion of the particles, and that severing the coal, gives it a friable 

 tendency. The Spadra coal, in common with the semi-anthracites of the 

 Shamokin coalfield of Pennsylvania, possesses this peculiar subdivision into 

 cuboidal lumps, indicative of a quicker escape of the expansible gases 

 than would take place under prolonged chemical evolution. 



This question of a former subterranean igneous action is interesting, not 

 only in its relation to the influence it may have exerted upon the coals of 

 the Arkansas valley, but also, in its important bearing upon the metalliferous 

 character of the underlying geological formations ; since it is a matter of 

 experience, that rocks are more apt to be intersected by metallic veins in 

 districts adjacent to axes of dislocation ; and these are a frequent accom- 

 paniment of subterranean igneous action. 



* It is likely that granite or some other igneous rock may be found in Montgomery county. 



