OF BUILDING STONES. 309 The most pleasing colours for building purposes are pale yellow, or light red. Perfectly white stones are not the most agreeable to the eye. Porphyries, and granites, generally retain their colour under exposure to the atmosphere. Chemical Characters. The chemical consti tuents of rocks are of great importance in reference to their durability under certain circumstances of the atmosphere, and conditions of climate. Fortunately, it is by no means difficult, for ordinary purposes of construction, to ascertain with sufficient accuracy the presence, or absence, of special minerals. The generality of building stones range themselves under the heads of sandstones and limestones; and the purest kinds of these rocks are those which con tain the smallest amount of foreign ingredients. But between these extreme cases, there are' to be found a great variety of intermediate kinds of rocks, generally called ‘ calcareous sandstones,’ in which the basis is composed of sand, or siliceous particles, with an intermixture of lime ; or ‘ siliceous limestones,’ in which the basis is carbonate of lime, with a pro portion of siliceous material; and it is frequently of importance to ascertain whether a sandstone contains any, or a large, proportion of carbonate of lime. This is easily effected by the simple use of dilute hydro chloric, or nitric acid. If the stone contains car bonate of lime or magnesia, effervescence will ensue
