3. _Pseudomorphous Carbonate of Lime._--This form of carbonate of lime occurs in Pope County, Illinois, a district celebrated for its fluorspar, lead, crystallized quartz, &c., and bearing the unequivocal marks of a secondary formation. Scattered in large masses over the soil, we observe compact limestone, with very perfect cubical, octahedral, or other regular cavities, which have manifestly originated from crystals of fluorspar. The most common _impress_ of this kind appears to have resulted from two cubes variously joined--a form of appearance very common to the Illinois fluates. Some of these cubical cavities exceed three inches square; but in no case is any remaining portion of the spar in these cavities, or anywise connected with the fragments of limestone thus impressed, although, at the same time, the spar is very abundant in the alluvial soil where these curious limestones are found. 2. SULPHATE OF LIME. Subs. _Gypsum._ 1. _Fibrous Gypsum._--In the alluvial soil of the St. Martin's Islands, Lake Huron. The fibres are sometimes five or six inches in length, of a white color and delicate crystalline lustre. Sometimes these fibrous masses are partially colored yellow or brown, apparently from the clay, or mixed alluvion, in which they are imbedded. 2. _Granular Gypsum._ } 3. _Granularly-Foliated Gypsum._ } With the preceding. 4. _Earthy Gypsum._ } 3. FLUATE OF LIME. _Fluor-Spar._--On the United States Mineral Reserve, Pope County, Illinois. This locality is abundant, and the mineral readily and constantly to be obtained. I first obtained specimens in June, 1818, and afterwards visited it in July, 1821. It is disseminated in loose masses throughout the soil, and in veins in the calcareous rocks. The spot most noted and resorted to, and where the original discovery was made, is four miles west of Barker's Ferry, at Cave-in-Rock, on the banks of the Ohio, and about twenty-six miles, by the course of the river, below Shawneetown. It is situated in the midst of a hilly, broken region, called _the Knobs_, a tract of highlands intervening between the banks of the Ohio and the Saline. The distance of this range from north to south, or parallel with the course of the Ohio, cannot be stated. It probably extends from near the banks of the Wabash River to the Little Chain of Rocks. Its breadth--from Barker's Ferry, west, to Ensminger's, at the Saline, is about twenty miles. It thus separates, by a rocky border, the prairies of the Illinois from the current of the Ohio River. These knobs, wherever observed, bear the indubitable marks of secondary formation, and may be stated to consist, essentially, of compact limestone resting on sandstone. The sandstone is sometimes so much colored by iron, and by globular or irregular masses of iron stone, as to give that rock a very singular aspect. This may be particularly instanced in the mural front of the Battery rocks on the banks of the Ohio.