I will now notice an important fact, applicable to the whole extent of limestone country, which has come under my observation. It is its association with a description of minerals, all of which appear to be _silicious_. To describe them minutely, would require several pages. From the time I entered the limestone country till I left it this association was observed. The minerals included in it differ much in their external character. Their size varies from that of rocks to the smallest fragments. Usually they lie loose upon the earth, in angular forms, having the appearance of a stone that has been broken in pieces by the hammer. Sometimes they cover the sides of hills and mountains in such abundance as to prevent or impede vegetation. When the disintegration is minute, they are serviceable rather than otherwise; and the farmer talks of his "good black," or "white gravel land." It renders this service, I presume, not by decomposition, but by preventing the soil and its manure from being washed away. Indeed the different varieties of it are generally scattered over the surface, in pieces so small, that for convenience sake, the whole may be denominated a _silicious gravel_. Sometimes the mineral is imbedded in limestone, in the form of nodules, thus indicating their original connexion with it. The varieties, so far as I have observed, are quartz, hornstone, flint, jasper, and semi-opal; and several, which to me are non-descripts. _Quartz_ is the most abundant. It is found of different colours; compact, and porous or cellular; of every size; simple and associated with other silicious stones; massive and crystallized. In Augusta and Rockbridge counties in Virginia, beautiful crystals of quartz, of a singular form, are found. They are six-sided prisms, with double acuminations, that is, with six-sided pyramids, mounted on the opposite ends of the prism. A specimen of two such crystals united, you have received. It was found near Lexington. A curious variety of the quartz gravel-stone occurs on both sides of Elk River, a few miles above its junction with the Tennessee, in the Alabama territory. As you travel to the west from Huntsville, it appears first in the neighbourhood of Fort Hampton, two miles east of Elk River, and may be seen for ten miles west of that river. The mineral is remarkable for containing a curious petrifaction. Its first appearance is that of a solid screw. On examination, however, you find it is not spiral; but consists of parallel concentric layers. Their diameter varies in different specimens, from that of a pin to half an inch. They stand in the centre of a hollow cylinder, extending its whole length, and occupying about one-third of its dimensions. The stone is sometimes perfectly filled with these forms. The petrifaction I could not have named, had you not pronounced it the "Entrochite."