Tripolite or Diatomaceous Earth.—This interesting rock is soft, light, and looks like clay; but it is lighter, and the argillaceous odor is faint or wanting. It does not effervesce with acid. Hence, it is neither clay nor chalk. Notwithstanding its softness, it is really composed of a hard substance, viz., silica, in the form known as opal. By rubbing off a little of the dust, and examining it under the microscope, we easily prove that the silica is mainly or entirely of organic origin; for the dust is seen to be simply a mass of more or less fragmentary organic remains, occurring in great variety, and of wonderful beauty and minuteness. There are few rocks so unpromising on the exterior, and yet so beautiful within. We have already learned that these organic bodies are principally Diatom cases, Radiolaria shells, and Sponge spicules. We can form some idea of their minuteness from Ehrenberg’s estimate that a single cubic inch of pure tripolite contained no less than 41,000,000,000 organisms. The lightness of tripolite (sp. gr., 1-1.5) is due to the facts that opal is a light mineral (sp. gr., 1.9-2.2), and that many of the shells are hollow. Tripolite is a good example of a soft rock composed of a hard mineral; and it owes its value as a polishing material to the fact that it consists of a hard mineral in an exceedingly fine state of division. Tripolite, when pure, is snow-white; but it is rarely pure, being commonly either argillaceous or calcareous. This rock is now forming in thousands of places, in both fresh water and the ocean. Flint and Chert.—During the course of geological time, beds of tripolite are gradually consolidated, chiefly by percolating waters, which are constantly dissolving and re-depositing the silica; and, finally, in the place of a soft, earthy rock, we get a hard, flinty one, which we call _flint_ if it occurs in the newer, or _chert_ if it occurs in the older, geological formations. Besides forming beds of nearly pure silica, which we call tripolite, the microscopic siliceous organisms are diffused more or less abundantly through other rocks, especially chalk and limestone. In such cases the consolidation of the silica implies its segregation also; _i.e._, the silica dissolved by percolating water is deposited only about certain points in the rock, building up rounded concretions or nodules. Thus, a siliceous limestone becomes, by the segregation of the silica, a pure limestone containing nodules of chert, which are usually arranged in lines parallel with the stratification. Specimen 16 is a fragment of a flint-nodule from the chalk-formation of England.