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ORIGIN, MATERIALS, &C. OF ROCKS. I 93 Besides these distinctions, the earths are sometimes formed into separate minute bodies, or minerals, which are again united so as to constitute rocks; and these may be, in themselves, either simple or com pound minerals. Sandstone offers an example of a simple rock of this kind; simple in its chemical nature, but an aggregate as to its general character. Hornblende rock is an example of an analogous aggregate, but one in which the integrant minerals are chemical compounds. But there are differences here, even in the mode of aggregation; which, in some cases, result from the chemical interference of a simultaneous crystallization, in others, from the mere mechanical approximation of the parts, and lastly, from the union of those two processes. Gra nular limestone is an example of the first; and in stances of the last are to be found in different varieties of sandstone. In compound rocks, different kinds of minerals are visibly united into a common mass; which thus pre sents a sort of uniformity throughout the whole, how ever the separate parts may differ. Such compounds may consist of two or more minerals; and, within certain limits, they seem to be ruled by laws as general as the simpler rocks. These compounded rocks vary, like the former, in being purely crystalline, or otherwise; and as granite presents a familiar ex ample of the first, so quartz rock, and some of the compound argillaceous schists, afford instances of the other two. I here is still another description of compound rocks, to which the term conglomerate has been applied. In these, not only different minerals are united in a mechanical, a mixed, or a chemical manner, but fragments of former rocks, either simple or com- VOL. I. o