_Petrosilex._--In the vicinity of Bainbridge, ten miles above Cape Girardeau, is a stratified gray flint rock very similar in aspect, and having nearly a similar fracture to the common gun-flint. This rock is here an extensive stratum, and occurs in connection with compact limestone. In tracing it towards the south-west, we have not been able to detect the slightest interruption to its continuity through an extent of more than two hundred miles along the central portion of the mountainous district. Towards the south-west it is found to acquire gradually a more and more primitive character, and losing, near the Chattahoocke mountain the accompanying stratum {304} of compact limestone, it appears near the hot springs of the Washita, associated with the highly inclined argillite of that district. This rock, as far as our limited observations have extended, exhibits no traces of organized remains. Its colour seems gradually to change according to its age, or at least with the apparent age of the rocks associated with it. South of the Arkansa it is of a yellowish or pearly white colour; about White river, it is a dirty yellow, and at the St. Francis a grayish brown. A corresponding change may also be noticed in the inclination of the strata, and in other particulars. Aside from this apparently intimate connection there is a particular resemblance between the petrosilex of the Washita, and the flint rock of the lead-mine district. The rock in both instances falls readily into small masses of a few ounces weight. The hills it forms have usually a rounded outline, and often bear open forests of pine, while the timber on the sandstone hills is usually oak. Open woods of pine and oak occur in almost all the uplands in the Ozark mountains, and are considered unfailing indications of a meagre and flinty soil. _Argillaceous Sandstone._--The sandstones of this small group of mountains appear under almost every variety of character, but in most of them, as far as hitherto examined, we discover traces of coal or of those minerals and organized remains which usually accompany it. In the inclined sandstone near the hot springs, there are, it is true, no indications of coal; and that rock is in every respect similar to what are called the transition sandstones of the Alleghany and Coatskill mountains, but by following it an inconsiderable distance either east or west, it is found passing imperceptibly into the coal strata of the Poteau, and of the Little Red river of White river. In this instance, as in that of the stratum last mentioned, we find a rock apparently possessing as much unity as can belong to such a subject, passing from {305} recent _secondary_ down, through all the intermediate grades, to the oldest _transition_, and thus heaping confusion upon our doctrines of the original continuity and systematic succession of strata.