There are many varieties of sandstone depending upon differences in composition, texture, etc., but we have not space to notice them in detail. In sandstone, just as in sand, quartz is the predominant constituent, although we sometimes find varieties composed largely or entirely of feldspar, mica, calcite, or other minerals. Specimen 31 is an example of the architectural variety known as freestone, which is merely a fine-grained, light-colored, uniform sandstone, not very hard, and breaking with about equal freedom in all directions. The consolidation of sandstones is due chiefly to chemical action. The cementing materials are commonly either: _ferruginous_ (iron oxides), giving red or brown sandstones; _calcareous_, forming soft sandstones, which effervesce with acid if the cement is abundant; or _siliceous_, making very strong, light-colored sandstones. Ferruginous sandstones are the most valuable for architectural purposes; for, while not excessively hard, they have a very durable cement. Siliceous sandstones are too hard; and the calcareous varieties crumble when exposed to the weather because the cement is soluble in water containing carbon dioxide, as all rain-water does. Specimen 32 is a good example of a ferruginous sandstone, and it is coarse enough so that we can see that each grain of quartz is coated with the red oxide of iron. The mica scales visible here and there in this specimen are interesting as showing that the grains are not necessarily all quartz; and it is important to observe that the mica was not made in the sandstone, but, like the quartz, has come from some older rock. Quartzite.—This rock is simply an unusually hard sandstone. Now the hardness of any rock depends upon two things: (1) the hardness of the individual grains or particles; and (2) the firmness with which they are united one to another. Therefore, the hardest sandstones must be those in which grains of quartz are combined with an abundant siliceous cement; and that is precisely what we have in a typical quartzite, such as specimen 33. Quartzite is distinguished, in the hand-specimen, from ordinary quartz by its granular texture (compare specimens 15 and 33); and of course in large masses the stratification is an important distinguishing feature.