reous, or glassy, texture is one of comparatively infrequent occurrence, but is represented by obsidian (q.v.) and pitchstone. It is now generally recognized that many igneous rocks which have an entirely crystalline texture were once largely composed of rock glass, which has devitrified through processes of chemical alteration and crystallization.

In addition to the above textures characteristic of igneous rocks and dependent upon the state of crystallization of the rock substance, there are other features which call for notice. One of the most common observed in lava (q.v.) is a peculiar crumpled lamination of the rock caused by the arrangement of mineral constituents of unequal dimensions with their longer axes parallel to the crumpled lines. This fluxion structure is conditioned by the flow of the lava, the crumpled lines indicating the direction of flow. Other structures, as the amygdaloidal, scoriaceous, and pumiceous, which indicate diferent grades of cellular or porous texture, are conditioned by the steam once held in the lava and the opportunities for expansion and escape of this steam as the lava approached the surface of the earth.

In chemical composition the igneous rocks show wide variation, though limits are set by the laws of formation of magmas. No such limits are set for the clastic rocks. By processes of alteration the igneous rocks change into the metamorphic rocks (as do also the clastic rocks), so that no sharp line can be drawn to separate these great divisions.

Great difficulties arise in the systematic classification of igneous rocks by reason of the many ways in which it is possible to describe them, and the varying importance which is attached to each method; hence great difference of opinion exists among petrographers as to the best scheme to be adopted. On the continent of Europe, and especially in Germany, the manner of occurrence of a rock, whether as a boss or batholite (q.v.), as a dike (q.v.), or as a sheet or flow, is given the first importance; and the age of the rock, once considered paramount, still retains its importance in the secondary classification. In America opinion seems to favor as bases of classification the chemical composition, the texture, and the mineral composition; but the future must be awaited before any classification free from serious objections is likely to be adopted. The scheme which has the greatest number of followers among recent workers in petrography perhaps is the so-called quantitative classification proposed by Iddings, Cross, Washington, and Pirsson, and described in the Journal of Geology for 1902. The basis of this classification is chemical composition, and rocks of similar chemical character are grouped together. The chemical composition is expressed in terms of certain standard minerals, which are divided into two groups, the alkali-calcic and the iron-magnesia groups. The rocks are separated into five main classes according to the relative proportions of the minerals present belonging to the two groups. The principal defect of the scheme from a practical standpoint seems to be that an exact chemical analysis is a necessary preliminary to placing a rock in its proper class.

Bibliography. Kemp, Handbook of Rocks (New York, 1904); Rosenbusch, Mikroskopische Physiographie der Mineralien und Gesteine (2 vols., Stuttgart, 1904-08); Zirkel, Lehrbuch der

:

in

Petrographie (Leipzig, 1894) Teall, British Petrography (London, 1888); Cross and others, "A Classification of Igneous Rocks," in Journal of Geology, vol. x, part ii (1902); Harker, The Natural History of the Igneous Rocks (New York and London, 1909); Pirsson, Rocks and Rock Minerals (New York, 1911). See GEOLOGY. IG'NIS FATUUS, făt'û-ús (Lat., vain fire). A luminous appearance, frequently seen marshy places, in churchyards, and over stagnant pools. It generally appears a little after sunset as a pale, bluish-colored flame, varying in size and shape; sometimes it shines steadily till morning, at other times disappears, and reappears within about half-hourly intervals. It floats in air at about 2 feet from the ground, is sometimes fixed, and sometimes travels with great rapidity. Many efforts have been made to discover its cause; but so varied are its appearances and so void of any common principle that these attempts have failed. Two of the various explanations offered may be mentioned here. The first is that the ignis fatuus is due to phosphureted hydrogen gas, PH, which possesses the property of igniting when it comes in contact with dry atmospheric air; the gas would be generated by the decomposition of animal matter present in a marshy soil. The second is that it is due to the combustion of methane, or marsh gas, CH, produced by the decomposition of vegetable matter; but though this supposition satisfactorily accounts for many appearances connected with the ignis fatuus, the gas itself is not spontaneously combustible, and an additional supposition becomes necessary to account for its ignition. The probable conclusion is that a number of phenomena, apparently similar, but arising from different causes, are aggregated under the term "ignis fatuus."

The appearance of ignis fatuus is not a common phenomenon, and many distinguished naturalists who desired to investigate it have never succeeded in finding it; but it is not unfrequently seen in the north of Germany, the swamp and moorland districts in the south and northwest of England, and in the lowlands of Scotland. It is seen in the above places from the middle of autumn till the beginning of November. In former times the ignis fatuus, under the names of Will-o'-the-Wisp, Jack-o'-lantern, Spunkie, etc., was an object of superstition among the inhabitants of the districts where it appears and was believed to be due to the agency of evil spirits attempting to lure travelers to their destruction.

IGNITION. See INTERNAL-COMBUSTION ENGINE and MOTOR VEHICLES.

IGNJATOVIĆ, ig-nyät'o-vich, Iakša (Jacob) (1824-89). A Servian novelist and critic, born in Szent-Endre, Hungary. He received a legal education at Budapest and elsewhere. After the revolution of 1848 he lived for some time in Paris, and he traveled extensively in south Europe. Returning to Hungary, he became editor of the Review (Letopis) of the Servian Literary Society (Matica) and was twice elected deputy to Parliament. He wrote numerous novels, historical, humorous, and social, of which the best are George Brankovich (1859), historical, and The Sufferers and Old and New Masters, dealing with contemporary Servian life. He also wrote short stories and political and literary

essays.

IGNORANCE OF THE LAW. One of the important maxims of the common law is Igno

rance of the law excuses no one. It is not, however, an aphorism of universal application, nor is it to be pressed into the service of injustice; neither does it warrant the presumption that every one knows the law. There is, of course, no such presumption. Yet the rule that ignorance of law shall not serve as an excuse for illegal acts or omissions is common to all systems of law and is not only expedient but necessary.

In criminal law particularly the maxim gives expression to an important principle of public policy. To permit a person who has committed a crime to escape the penalties of his wrongdoing by the plea that he did not know that the act was a crime, would plainly be subversive of the whole system of criminal justice. Yet even in criminal cases the principle has real or apparent exceptions. If a person is indicted for larceny, he may show that he honestly believed the property to be his own, although it appeared that this belief was due to his ignorance of a rule of law which vested the property in another. This results from the fact that he does not commit the crime of larceny unless he intended to appropriate another person's property to his own use. Had the indicted person been sued for conver-. sion by the true owner, his ignorance of the law would have been no excuse. In the civil action for conversion his motive, or intention, or belief in taking would be immaterial. One who takes and uses property as his own acts at his peril. So, if a person enters into an honest and fair contract, he cannot absolve himself from its obligations by showing that he would not thus have contracted had he known the legal consequences of so doing.

In general it may also be said that a person who has paid money in ignorance of the law cannot recover it in a quasi-contract action as money paid under a mistake. This means that no man will be permitted to exempt himself from a duty, or shelter himself from the consequences of infringing a prohibition imposed by law, or acquire an advantage in opposition to the legal rights and interests of another, by pretending error or ignorance of law. See MISTAKE; QUASI CONTRACT.

IGNORAN'TINES (from Lat. ignorare, to be ignorant), FRATRES IGNORANTIÆ. A religious congregation of men in the Roman Catholic church, associated for the gratuitous instruction of poor children in sacred as well as secular learning. See BROTHERS OF THE CHRISTIAN SCHOOLS.

IGOROT, i'gô-rot'. The designation Igorot is a term meaning 'mountain people,' which was formerly applied to all the interior people of northern Luzon. At present it is generally understood to refer to the pagan inhabitants of Bontoc, Benguet, Lepanto, and Amburayan subprovinces. In culture these people are much alike except for the fact that those outside of Bontoc have been more modified by the white man. Both are agricultural and depend almost solely on the fields for sustenance. Like the Ifugao, they construct mountain-side terraces, to which they conduct water by means of flumes and ditches. The houses, which are grouped in villages, have grass roofs which overhang the board side walls. The ground floor is used as a sleeping apartment and general workroom, while in the roof is a dimly lighted apartment which serves as a kitchen and storeroom.

The people of both divisions are of excellent build, but the inhabitants of Bontoc are some

what larger and more aggressive than the Benguet-Lepanto people. The Bontoc are addicted to head hunting and until recent years have been ardent warriors. Their villages are divided into atos (or divisions), in each of which is to be found a special house for the unmarried men and boys, while a similar structure serves for the girls. The Philippine census of 1903 stated the total number of the Igorot people as 211,520.

While less aggressive than the people of Bontoc, the Benguet-Lepanto Igorot have developed further in many ways, and are now, as they have been for many centuries, miners and workers in gold and copper. Consult Jenks, The Bontoc Igorot (Manila, P. I., 1905). See PHILIPPINE ISLANDS.

IGOR'S (ĕ'gôrz) BAND, SONG OF (Russ. Slovo o polku Igorevě). An early Russian prose narrative, of uncertain date. It recites the unfortunate outcome of the expedition (1185) of Igor (Scand. Ingvar), Prince of Novgorod-Syeversk, into the country of the Polovtsy in southern Russia. The Prince was defeated and taken prisoner, but later he escaped with the help of a slave. This song, so called, is not contemporary with the events it relates. Generally admired are the lyric passages depicting the grief of Nature over the Prince's capture; the lament of Yaroslavona, his consort, on hearing of his fate; and the escape of Igor. The original manuscript, discovered by Count Musin-Pushkin and published in 1800, was destroyed in the Moscow fire of 1812. The editing was very poor, and another copy found in 1864 among Catharine II's papers is hardly more satisfactory. Its authenticity has been an object of bitter controversy, and a vast literature on the subject has grown up. A translation by Wolfsohn in Schönwissenschaftliche Litteratur der Russen (Leipzig, 1843) and one with introduction and notes by Paucker (Berlin, 1884) are very good. There are also French translations by Rambaud (La Russie épique, Paris, 1876) and others. There is an English translation by Wiener in Anthology of Russian Literature, vol. i (New York, 1902).

IGRAINE, i-gran'. See IGERNA.

IGUALA, or IGUALA DE ITURBIDE, -gwä'là dâ e'toor-be'på. A town in the State of Guerrero, Mexico (Map: Mexico, J 8). It is in a rich mining and agricultural district and is the largest town of the state. The first silver mines worked by the Spaniards in Mexico were near here. It has a prominent place in Mexican history from the fact that the Plan de Iguala, or the Plan of the Three Guarantees, was announced here by Iturbide on Feb. 24, 1821. (See MEXICO, History.) In 1907 a severe earthquake destroyed many buildings in the town. Pop., 1910, 8195.

IGUALADA, ē’gwȧ-lä’Då. A city in the Province of Barcelona, Spain, situated on the river Noya, in a mountainous but fertile district, 46 miles by rail northwest of Barcelona (Map: Spain, F 2). It was formerly a fortified town, but its walls are now in ruins. It has manufactures of cotton, linen, and other textiles. The famous mountain and Benedictine monastery of Monserrat lie 12 miles to the east. Pop., 1900, 10,476; 1910, 11,754.

IGUANA, i-gwa'nå (Sp., from the Haitian name, igoana, hiuana, yuana). A genus of tropical American lizards, representing the family Iguanidæ, of which there are about 55 genera and 250 or more species. In external and internal structure iguanas closely resemble the Agamida

of the Old World and are distinguished mainly by the pleurodont dentition. The tongue is thick and villous. All the North American forms possess femoral pores, but few of the South American species have them. In habits also the Iguanidæ closely resemble the Agamidæ, save that

TEETH OF AN IGUANA.

there are no flying forms to correspond with the flying dragon of the Agamida, while in America some of the iguanas, such as Anolis, have digital expansions, and others are semimarine, neither of which conditions is met with in the Old World family. The family contains several of the largest lizards. Most of the species are arboreal, but some of them live on the surface of the sand and stones of the desert and have a depressed form. Several species live wholly, or almost wholly, on vegetable food-the blossoms and leaves of plants. For this reason the flesh of several of them, especially of the genus Iguana, is very palatable, is sought by the natives of Central and South America as food, and is sold in their markets in considerable quantity. One of the species most eaten is Iguana tuberculata, a repulsive-looking lizard, with a high, dorsal, fringed ridge and a large dewlap. (See Plate.) It basks on the limbs of trees during the warm hours and while thus situated seems rather indifferent to the approach of man. It is fond of music and of having the body stroked. The natives take advantage of these facts, whistle a lively tune as they approach, and when near enough stroke the sides of the iguana with a stick until they succeed in getting a noose over its head. (Consult Belt, Naturalist in Nicaragua, London, 1888.) The natives also dig them out of their burrows or chase them into trees with dogs trained for the purpose. On the Galapagos Islands there is a semiaquatic genus, Amblyrhynchus, whose species feed on seaweeds along shore. This lizard is described in detail by Darwin, in chap. v of his Naturalist's Voyage (London, 1860). There are other species of the same genus that live for months without water by feeding on the succulent cactus. The great iguana of Jamaica, with the prominent serrate crest, is Cyclura lophoma. In the southwestern United States, from western Kansas to southern California and Mexico, dwell several genera of green, dark-gray, or brown iguanid lizards, such as Uta, Holbrookia (q.v.), Ctenosaura, and Crotaphytus. (See COLLARED LIZARD.) The large genus Sceloporus ranges not only over the western and central parts of the United States, but in all the eastern and Gulf States as far north as New Jersey and Indiana. Another widely distributed genus, Anolis, has two representatives in the South Atlantic States, popularly called chameleons. (See ANOLIS.) These lizards possess mimicry of color in a remarkable degree and have a considerable power of changing their color. They are insectivorous,

and the wanton destruction of them is much to be deplored, for in their native habitats they are of considerable economic importance to agriculture. To this family belongs also the basilisk of Central and South America, so named on account of its fancied resemblance to the creature

of fable. (See BASILISK.) It is a large, harmless lizard, found no farther north than southern Mexico. The family also includes those peculiar, spiny, short-tailed, flattened lizards known as horned toads. (See HORNED TOAD.)

Consult: G. A. Boulenger, Catalogue of Lizards of the British Museum (London, 1885); J. S. Kingsley (ed.), Standard Natural History, vol. iii (Boston, 1885); L. H. Stejneger, Death Valley Expedition (Department of Agriculture, Washington, 1893); E. D. Cope, Crocodilians, Lizards, and Snakes (Smithsonian Institution, ib., 1900); Hans Gadow, Amphibia and Reptiles (New York, 1901). See LIZARD.

IGUANODON, i-gwǎn'ô-don (Neo-Lat., from A Eng. iguana + Gk. ódoús, odous, tooth). genus of ornithopod dinosaurs, found fossil in Jurassic and Lower Cretaceous rocks of Europe. Iguanodon is perhaps the first dinosaur brought to light, for it was described by Mantell in 1825 from specimens found in Kent, England. The animal was 15 to 25 feet long; the head was large and narrow, the jaws heavy and furnished with numerous teeth of peculiar form that resemble those of the modern American lizard, Iguana. The anterior portions of the jaws are provided with strong, horny beaks like those of turtles. The fore limbs have four toes and a "spur" and are much shorter than the heavy three-toed hind limbs. The pelvis is elongated and the tail heavy, and there is a ridge of strong vertebral spines extending along the dorsal surface from the neck to the middle of the tail. All the bones are hollow. Iguanodon was an herbivorous dinosaur that walked on its hind legs and sat on a tripod formed by the hind legs and tail, after the manner of the kangaroo. It lived in great numbers in the swampy regions of England and Belgium and other parts of Europe during Jurassic time, as indicated by the mode of occurrence of its fossil skeletons, 29 of which were found at one time in Upper Jurassic sandstones of the coal regions at Bernissart, Belgium. Iguanodon has not been found in the Mesozoics of America, where it is represented by similar genera Camptosaurus, Laosaurus, and Thespesius. A very instructive account of the discovery and early history of Iguanodon, illustrating the method by which fragmentary material has after careful work yielded valuable results, is to be found in Mantell's Petrifactions and their Teachings (London, 1851). See also Woodward, Outlines of Vertebrate Palæontology for Students of Zoology (Cambridge, 1898). For more specialized articles, consult: Dollo, "Quatrième note sur les Dinosauriens de Bernissart," in Bulletin du Musée Royal d'Histoire Naturelle de Belgique, ii (Brussels, 1883); Woodward, Geological Magazine (London, 1895); Knipe, Evolution in the Past (ib., 1912). See CamptosauRUS; DINOSAURIA.

IGUAZÚ, é'guá-soo'. A river of Brazil, which rises near the Atlantic Ocean in the State of Paraná, flows westward for about 790 miles, and empties into the Paraná River (Map: Argentina, J 3). It forms most of the boundary between the states of Paraná and Santa Catharina and also for a short distance between Brazil and Argentina (Misiones). The middle course is navigable for river steamers for some distance. The lower course is broken by numerous cascades and falls, some being from 30 to 90 feet in height. About 11 miles from its mouth are the wonderful Falls of Iguazú, one of the most magnificent cataracts in the world.

1. IGUANA (Iguana tuberculata).

2. GREEN LIZARD (Anolis principalis).

3. NORTHERN HORNED TOAD (Phyrnosoma Douglassi); top of head and profile.

4. CHUCKWALLA (Sau.omalus ater).

5. LEOPARD LIZARD (Crotaphytus Wislizeni).

6. DESERT LIZARD (Callisaurus draconoides); head and hind limb.

7. GILA MONSTER (Heloderma horridum).

8. COMMON HORNED TOAD (Phrynosoma cornutum).