Third, and this is one of the most important facts in geology, all water-deposited sediments are arranged in horizontal layers, _i.e._, are stratified. And we have now traced to its conclusion, though very briefly, the process of the formation of one great division of _stratified_ rocks,—the _mechanically-formed_ or _fragmental_ rocks. These are so called because the clay, sand, and gravel are, in every instance, fragments of pre-existing rocks; and because the formation, transportation, and especially the _deposition_ of these fragments, are the work chiefly or entirely of mechanical forces. CHEMICAL DEPOSITION.—It is a well-known fact that the sea holds in solution vast amounts of common salt as well as many other substances; and analyses of river-waters show that dissolved minerals derived from the chemical decomposition of the rocks of the land are being constantly carried into the sea. Portions of the sea which are cut off from the main body, and which are gradually drying up, like the Great Salt Lake, Dead Sea, and Caspian Sea, become saturated solutions of the various dissolved minerals, and these are slowly deposited. This process is very nicely illustrated along our shores in summer, where, during storms, salt-water spray is thrown above the reach of the tides, and, collecting in hollows in the rocks, gradually dries up, leaving behind a crust of salt. When the sea lays down matter which it held in _suspension_, we call the process _mechanical_ deposition, and the result is _mechanically_-formed rocks. But when it lays down matter which it held in _solution_, we call the process _chemical_ deposition, and the result is _chemically_-formed rocks. The principal substances which the sea deposits chemically are common salt, forming beds of rock-salt; sulphate of calcium, forming beds of gypsum; carbonate of calcium, forming beds of limestone; and the double carbonate of calcium and magnesium, forming beds of dolomite. Inorganic deposition, like inorganic erosion, is both chemical and mechanical. 2. _Animals and Plants, or Organic Agencies._ We turn now to the consideration of the _organic_ agencies. And I will merely allude in passing to the vast importance of the fossil organic remains found in the stratified rocks as marks by which to determine the relative ages of the formations. As regards the _destruction_ of rocks—_erosion_—plants and animals are almost powerless; but in the role of _rock-makers_ they play a very important part, being very efficient agents of _deposition_. FORMATION OF COALS AND BITUMENS.—Specimen No. 8 is an example of peat from the vicinity of Boston; but just as good specimens may be obtained in thousands of places in this and other States.