APPENDIX D. — GEOLOGY. 141 



1. Sub-soil, arenaceous, and of a red color, three to ten feet. 



2. Black shale, soft, and rapidly disintegrating, four feet. 



3. Seams of bituminous coal, two to four feet. 



4. Fine-grained sandstone, yellowish gray, with fossil ferns ; thick- 

 ness variable. 



5. Gray non-fossiliferous limestone, of unknown thickness. 



Dr. Shumard says that the fossil ferns in this formation belong to 

 "the carboniferous era." He also describes the same formation on the 

 third day's march, some fifty miles northeast of Fort Belknap, on one of 

 the sources of Trinity river. He describes sandstone for several subse- 

 quent days, some of it coarse and highly ferruginous, with ripple-marks, 

 which I should suppose might belong to the same coal measures, did he 

 not mention that strata of red loam, so abundant in all that region, lie 

 beneath the sandstone ; which could not be, if the coal belongs to the 

 carboniferous period. Yet he mentions that the same formation as that 

 around Fort Belknap is largely developed between Fort Washita and 

 Fort Smith, on Arkansas river. The latter fort is not less than three 

 hundred and fifty miles northeast of Fort Belknap. On the 3d of May 

 he describes "large quantities of ironstone strewn over the surface," 

 another accompaniment of the true coal. 



Now, at first view it would seem almost certain that we have here a 

 description of a genuine coal formation of the carboniferous period, not 

 less than three hundred and fifty miles long, associated, moreover, with 

 those valuable iron ores which in other parts of the world are connected 

 with such deposites ; for, in descending through the formation, we find, 

 first, overlying shale, then coal, then coal sandstone, or perhaps millstone 

 grit, and then perhaps carboniferous limestone. But it is well known 

 that coal occurs in other rocks besides the carboniferous, as in Eastern 

 Virginia in oolitic sandstone, and in other places in tertiary strata. 

 These more recent coals are often of great value, as in Virginia ; but they 

 are not generally as good as those from the carboniferous strata. It 

 becomes an important question, therefore, to determine to what geologi- 

 cal period the coal under consideration belongs. A few specimens of the 

 fossil ferns would decide the matter, and I trust that Dr. Shumard is 

 •right in referring them to the carboniferous era ; but it is known that 

 analogous species occur in the higher rocks ; and so, coal, even in the 

 tertiary strata, is sometimes more or less bituminous. The evidence, 

 however, appears to me to be strong in favor of this deposite being of 

 the carboniferous age. But in your letter of April 1st, you state some 

 facts respecting this coal that have thrown a little doubt over my mind. 

 You say that — 



