CRYSTALLINE ROCKS 213 CTESIPHON on their surfaces, always keeping the characteristic angles exactly the same. Even when the growth of crystals in dif- ferent directions takes place, with un- equal rapidity, and distorted forms arise, as is often the case, the law still holds good, the inclination of the adjacent planes and the angles which they enclose are the same. Hence the importance and the value of crystallometry and the science of crystallography or crys- talology. Crystals occur with an almost infinite variety of forms — calcareous spar having alone more than 200 forms in more than a thousand different com- binations, and some crystals have as many as 300 different sides. But all crystals may be grouped in accordance with certain systems. CRYSTALLINE ROCKS, a name given to all rocks having a crystalline structure. The crystalline texture may either be original or superinduced. Thus some crystalline rocks, such as certain calcareous masses, owe their origin to chemical precipitation from water, while others again, such as lavas, have consoli- dated from a state of igneous fusion. There is another large class of crystal- line rocks, the crystalline granules of which present a remarkable foliated character — that is, they are arranged in more or less parallel layers. This pecu- liar schistose structure appears to have been superinduced — the original rocks having been either fragmental or crystal- line or both — and the result of greal; heat and pressure. Such highly altered rocks occur in the neighborhood of masses of granite, and cover wide regions, where there is abundant evidence to show that the strata have been subjected to enor- mous compression. It is therefore be- lieved that pressure and the heat engen- dered by great earth-movements, and the intrusion of plutonic igneous matter, are among the most potent agencies in the production of schistose structure. CRYSTALLITES, minute non-polariz- ing bodies (the result of incipient crys- tallization) occurring in the vitreous portions of igneous rocks. CRYSTALLOGRAPHY, the science which describes or delineates the form of crystals. In A. D. 1672, Rome de Lisle published his "Essay on Crystallog- raphy," but the honor of being regarded as the founder of the science is given to the Abbe Rene-Just Haiiy. He was born at St. Just, in what is now called the department of Oise, and, among other works, published his "Essay on the Structure of Crystals," in 1784, as also his "Treatise on Mineralogy" and his "Treatise on Crystallography," both in 1822 — the year of his death. His view was that all the varieties of crystals which a particular mineral may assume are derivable from one simple form, which is the type of the mineral. That form he attempted to ascertain in each individual case. Essentially the same view is still held. Imaginary lines may be supposed to be drawn through a sim- ple crystal longitudinally from end to end, transversely from side to side, or in either of those ways, or obliquely from angle to angle, around which imaginary lines all the particles of matter compos- ing the crystal may be supposed to ar- range themselves. Such imaginary lines are called the axes of the crystal. If skillfully chosen they become somewhat more than imaginary lines, for they may coincide with the optical axes of the crystal if it possess double refraction. According to the number, relative length, position, and inclination to each other of these lines depends the outward form of the ci'ystal. Dana enimierates the following "sys- tems of crystallization": (1) Having the axes equal — ^the Iso- metric system. (2) Having only the lateral axes equal — the Tetragonal and Hexagonal systems. (3) Having the axes unequal — the orthorhombic, monoclinic, and triclinic systems. See Crystal. CTENOPHORA, an order of Actinozoa, consisting of marine animals which swim by means of ctenophores. The body, which is gelatinous and transparent, is generally more or less oval in form. Most of the species have a pair of very extensible filiform tentacles. There are two tribes, eurystomata and stenostovw- ta, the first containing the family heroidse, and the second the families saccate, lohatas, and tasniatas. The ctenophora are found in all seas. CTESIAS, a Greek historian of about 400 B. C, contemporary with Xenophon and partly with Herodotus. He was a physician, and lived for 17 years at the court of Persia. He wrote a "History of Persia," of which little remains. CTESIPHON, a city of Babylonia, on the E. bank of the Tigris and opposite Seleucia, the common winter residence of the Parthian kings, and finally the capital of the Pai'thian kingdom. It was conquered by the Romans in A. d. 115, and destroyed by the Arabs under Omar in 637. Its ruins still attest its former magnificence. CTESIPHON, BATTLE OF, an im- portant military engagement, fought on Nov. 22, 1915, between Turkish forces
