APPENDIX D. GEOLOGY. 151 



From your description, especially in 'your lecture before the American 

 Geographical and Statistical Society, it is manifest that the character o 

 the rocks changes on the northwest of the gypsum formation, and near 

 the head of the south branch of Red river. The red clay and gypsum 

 have disappeared, and sandstone succeeds ; but of what age I have no 

 means of judging. 



Another interesting mineral found by you in the red clay and gypsum 

 formations above described, is copper. The specimens were put into 

 the hands of Professor Charles U. Shepard, who has analyzed them, as 

 well as several other specimens, in the laboratory of Amherst College 

 and whose report I annex to. my own. You will see that he has made 

 free use of your name by attaching it to a new ore of copper, found on 

 Red river near the Witchita mountains ; and that he describes three or 

 four other species of copper ore from the same region. For a particular 

 description I refer you to his report, while I confine myself to a few re- 

 marks as to the geology of the deposite. 



On section V, Dr. Shumard has shown the geological position of this 

 ore, viz : near the bottom of, and in the red clay, and more than one 

 hundred feet from the surface. We hence see tbat the ore was deposited 

 from water, although some specimens from Cache creek of calcareous 

 amygdaloid seem to have been melted. But if, as has been suggested, 

 the gypsum was produced by the joint action of submarine volcanoes 

 and water the copper may have had the same origin, and this would 

 explain the presence of chlorine in the Marcylite. 



How much copper niay be expected in such a region as that on Red 

 river, I have no means of judging, because I know of no analogous 

 formation. But as we have proof that it is an aqueous deposite, and 

 that igneous agency has been active not far off, it would not be 

 strange if the vicinity of the Witchita mountains should prove a prolific 

 locality. 



The oxide of manganese described by Professor Shepard may, per- 

 haps, be found abundant and more pure. And the iron-sand, so common 

 in some of the creeks, indicates the existence of magnetic oxide of iron 

 in the mountains. 



Whether the red clay formation and the gypsum formation that have 

 been described are of the cretaceous age or not, there can be no doubt 

 as to the deposites passed over from July 20 to Fort Washita, for among 

 the specimens are two species of Gryphcea, and one echinoderm, much 

 mutilated, but evidently of that period. On the 27th, a "bluish gray, 

 highly crystallized limestone" was observed, which cropped out be- 

 neath the sandstone, and which Dr. Shumard says was " non fossil- 



