Sienite was much used by the ancients in ornamental architecture. What was called the red granite of Egypt (for this rock has usually been considered a granite) furnished numerous magnificent obelisks and columns, of a single piece, which have been much admired in Rome and other places. The ancient artists sometimes cut this kind of stone into statues, vases, monumental and other works. The celebrated column in Egypt, upwards of ninety feet high, and known by the name of Pompey’s Pillar, is formed of sienite. In veins of this rock are found, in different countries, many kinds of metallic ores: among others, silver, iron, tin, copper, and lead. 263. _QUARTZ ROCK is a simple mountain rock, usually of granular texture, and whitish colour._ _It sometimes contains mica, in which case it has a slaty form._ In certain mountains of Scotland, and the Scottish islands, quartz rock is very abundant. On the Continent it appears in Saxony, Bohemia, Silesia, and several other countries. We are informed that a mountain, 350 feet high, and near 5000 feet broad and long, one of the Altaisch chain, in Siberia, consists entirely of a milk─white quartz. The uses of quartz have been already described (76, &c.) This kind of rock does not contain metallic ores of any description. II. SECONDARY ROCKS. 264. SECONDARY ROCKS _are composed of, or at least contain within them, the mineralized remains of organic substances_. These must necessarily have been formed at a period subsequent to the formation of those organized bodies the remains of which they enclose; and they have apparently been formed by the deposition of water. Hence it is that, to distinguish them from rocks of the preceding class, they have received the appellation of secondary. They always rest upon or cover primitive mountains, and sometimes lean upon their sides or invest them. Werner, the celebrated German mineralogist, makes two divisions of secondary rocks. The first of these he denominates _transition rocks_, and states that they are less perfectly crystallized than the primitive rocks; and that they enclose the remains of marine animals, no species of which are at this time known to exist: the other division he terms _floetz_, or _flat rocks_, because they are generally disposed in horizontal or flat strata. Some of the latter contain the fossil remains of marine animals and shells, approaching in character and appearance to the kinds which are now found in the ocean; and others contain shells precisely similar to those now known to exist. These rocks usually occur at the foot of primitive mountains, or in deep valleys. 1. TRANSITION ROCKS. 265. _TRANSITION LIMESTONE is distinguished by containing marine petrifactions of corals, and other zoophytes which are supposed no longer to exist. It often contains veins of calcareous spar, and exhibits a variety of colours, which give to it a marbled appearance._