ENTLiciLl GEOLOGY SAN LUIS DISTRICT, SECTION A. 311 



The sandstones extend down to the Ai^kansas, form the bed of the 

 river for some distance, and are again found on the other side. Pro- 

 ceeding to the consideration of the strati graphical and lithological con- 

 ditions observed in this formation, I refer to section A, Plate VIII, 

 Eeclining- upon the reddish Silurian sandstone, and conformable with 

 it, is a thick stratum of yellow and brown sandy shale (d) covered by 

 light-yellow and gray shales. These two strata weather very regularly, 

 forming a gentle slope, which is broken by the overlying limestone, (e,) 

 giving occasion to the formation of a gradually-rising bluff. On expos- 

 ure, this limestone weathers white, while its color on a fresh break is 

 grayish blue. Quite near the top of the eminence formed by it, a few 

 fossils were obtained, representing Orthis, Prochictus^ and Orthoceras. 

 Overlying the limestone, we find shales of a gray and brown color, (/, ^,) 

 which in turn are succeeded by other shales, more sandy, however. All 

 of them weather quite dark, and produce a gently undulating surface, 

 which is brought to a close by a series of heavy strata of dark blue 

 limestones, (7^,) capped by brown sandstone, (i.) The latter is middle- 

 grained, partly argillaceous, and contains small aggregations of magne- 

 tite crystals. 



To show the position of the Carboniferous with reference to surround- 

 ing formations, a section may serve, running from station 46 in a course 

 of east 6° north for a distance of about twenty-one miles. (Section B, 

 Plate VIII.) 



Station 46 is located on the west side of the Arkansas, on a mountain 

 of probably Post-Silurian granite. Descending from that into the valley 

 of the Arkansas, we find the base covered with drift. On the east bank 

 of the river, a bluff of light-colored trachyte («) rises abruptly, forming 

 a small cone, the edges of which partially overflow the succeeding red 

 granite, which forms low hills, until the steep Silurian ledge is reached. 

 In detail of the Silurian I may refer to section A, Plate VIII. It has a 

 dip of 16° to 12° eastward, underlying the Carboniferous strata. For 

 the latter the character given in section A is applicable. Descending 

 to an elevation of about 9,360 feet, (camp 46,) the trough of a shallow 

 synclinal valley is reached, and from that point the strata begin to rise 

 again toward the east, gradually at first, increasing, however, to a rise 

 of 15°. The sandstone upon which station 54 was l&cated does not ex- 

 tend far enough north to be cut by this section. It was impossible to 

 determine whether the Silurian again outcrops at this eastern junction 

 of Carboniferous with the granite, but it seems highly probable. Gran- 

 ite that is lithologically identical with that between stations 51 and 53 

 appears east of the edge of the termination of sedimentary beds. Tra- 

 chorheites (e) form the last member of this section, having overflowed 

 a large portion of the granite, and not unlikely also some of the sedi- 

 mentary beds. 



Neariug Badger Creek from the west, the Carboniferous narrows with the 

 Silurian, while the sandstone of station 54 pinches out entirely. About 

 five to six miles southwest of station 56 a very interesting and in- 

 struotive point is found demonstrating the position taken by the volcanic 

 rock to sedimentary. The section, #a portion of which has been used 

 in to illustrate .the geognostic features of the Silurian, is taken so as to 

 ciit the points referred to. Overlying the yellowish shales, interstratified 

 with quartzites, which have been referred to the Upper Silurian, are 

 a series of blue limestones (/) irregularly alternating with gray shales 

 containing Orthis and crinoids. Possibly these shales mi ;'ht be regarded 

 as Devonian, but the absence of any tyi)ical fossils by which a correct 

 classification could be made, allows them to be placed either in that posi- 



