Conglomerate shows the same variations in composition and texture as gravel; it may be composed of almost any kind of material in pebbles of almost any size. We recognize two principal varieties of conglomerate based on the forms of the pebbles; if, as is usual, these are well rounded and water-worn, the rock is true _pudding-stone_ (specimen 29); but, if they are angular, or show but little wear, it is called _breccia_. (2) _Arenaceous Group._—The conglomerate group passes insensibly into the arenaceous group; for, from the coarsest gravel to the finest sand, the gradation is unbroken, and every sandstone is merely a conglomerate on a small scale. Sand.—Like gravel, sand may be of almost any composition, but as a rule it is quartzose; quartz, on account of its hardness and the absence of cleavage, being better adapted than any other common mineral to form sand. Where the composition of a sand is not specified, a quartzose sand is always understood. By examining a typical sand with a lens, and noting the glassy appearance of the grains, and then testing their hardness on a piece of glass, which they will scratch as easily as quartz, the pupil is readily convinced that each grain is simply an angular fragment of quartz. Specimen 30. Sandstone.—Consolidated sand. In proving this, children will notice first the granular or sandy appearance of the sandstone; and then, with the lens, that the grains in the sandstone have the same forms as the sand-grains. The stratification cannot be seen very distinctly in our hand-specimens, but in larger masses it is usually very plain, as may be observed in the blocks used for building, and still better in the quarries. However, even if the stratification were not visible to the eye, we could have no doubt that sandstone is a mechanically formed stratified rock; because the form of the grains, just as in the conglomerate, tells us that. Many sandstones, too, contain the fossil remains of plants and animals, and these are always regarded as affording positive proof that the rocks containing them belong to the aqueous or stratified series.