APPENDIX D. GEOLOGY. 159 



report on the palaeontology of the expedition. We saw here some 

 specimens of ammonites several feet in diameter, and weighing between 

 four and five hundred pounds. On Red river, twenty-six miles from 

 Fort Washita, the sandstone of the cretaceous group supports about 

 twenty-five feet of ash-colored calcareous loam, which, on inspection, 

 was found to contain terrestrial and fluviatile shell of the genera Lymnea, 

 Physa, Planorbis, Pupa, aud Helix, the whole resembling species which 

 we have observed in the loam at New Harmony, Indiana, and elsewhere 

 in the Mississippi valley, which Mr. Lyell, during his visit to this 

 country, recognised as the equivalent of the loess of the Rhine. 



The geological formation, as developed in the vicinity of Camp 

 Belknap, consists of nearly horizontal strata of fine-grained sandstone, 

 shale, and soft, drab-colored, non-fossiliferous limestone, whose relative 

 positions correspond with strata of the same character largely developed 

 between Fort Washita and Fort Smith. On the surface were in many 

 places strewn fragments of a reddish-gray, igneous rock, containing a 

 large per-centage of carbonate and oxide of iron. From the frequent 

 indications of the presence of that metal in various localities of this 

 region, it is not improbable that this may become hereafter an extensive 

 and profitable field of mining enterprise. Recently a number of seams of 

 bituminous coal, varying in thickness from two to four feet, as well as the 

 characteristic fossil ferns of the carboniferous era, have been discovered. 

 The following section, taken about one mile from the post, may give 

 a better idea of the formation : 



1. Subsoil arenaceous, and of a red color; thickness from three to 

 ten feet. 



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



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



4. Fine-grained sandstone, of a yellowish gray color, and containing 

 fossil ferns ; thickness variable. 



5. Gray non-fossiliferous limestone ; thickness unknown. 



The water obtained from springs in this vicinity frequently contains 

 iron in solution. I have been informed that in a few instances chloride 

 of sodium has been detected in it. 



May 3. — Formation the same as at Camp Belknap. Observed, 

 strewn over the surface, large quantities of iron-stone ; soil and subsoil 

 arenaceous, and deeply tinged with oxide of iron. 



May 4. — Saw a number of horizontal layers of coarsely laminated 

 sandstone ; between the laminations were observed a large number of 

 ripple-marks. Soil good, and of a dark color; subsoil, in some places, 

 arenaceous, in others argillaceous, and of a deep-red color. 



