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  The form of this letter which is familiar to us is an invention of the Romans, who had previously converted the third symbol of the alphabet into a representative of a k-sound (see ). Throughout the whole of Roman history C remained as the symbol for G in the abbreviations C and Cn. for the proper names Gaius and Gnaeus. According to Plutarch (Roman Questions, 54, 59) the symbol for G was invented by Spurius Carvilius Ruga about 293 This probably means that he was the first person to spell his cognomen RVGA instead of RVCA. G came to occupy the seventh place in the Roman alphabet which had earlier been taken by Z, because between 450 and 350  the z-sounds of Latin passed into r, names like Papisius and Fusius in that period becoming Papirius and Furius (see ), so that the letter z had become superfluous. According to the late writer Martianus Capella z was removed from the alphabet by the censor Appius Claudius Caecus in 312 To Claudius the insertion of G into the alphabet is also sometimes ascribed.

In the earliest form the difference from C is very slight, the lower lip of the crescent merely rising up in a straight line , but and  are found also in republican times. In the earliest Roman inscription which was found in the Forum in 1899 the form is written from right to left, but the hollow at the bottom lip of the crescent is an accidental pit in the stone and not a diacritical mark. The unvoiced sound in this inscription is represented by K. The use of the new form was not firmly established till after the middle of the 3rd century

In the Latin alphabet the sound was always the voiced stop (as in gig) in classical times. Later, before e, g passed into a sound like the English y, so that words begin indifferently with g or j; hence from the Lat. generum (accusative) and Ianuarium we have in Ital. genero and Gennajo, Fr. gendre and janvier. In the ancient Umbrian dialect g had made this change between vowels before the Christian era, the inhabitant of Iguvium (the modern Gubbio) being in the later form of his native speech Iuvins, Lat. Iguvinus. In most cases in Mid. Eng. also g passed into a y sound; hence the old prefix ge of the past participle appears only as y in yclept and the like. But ng and gg took a different course, the g becoming an affricate dẓ (dzh), as in singe, ridge, sedge, which in English before 1500 were senge, rigge, segge, and in Scotch are still pronounced sing, rig, seg. The affricate in words like gaol is of French origin (geôle), from a Late Lat. gabiola, out of caveola, a diminutive of the Lat. cavea.

The composite origin of English makes it impossible to lay down rules for the pronunciation of English g; thus there are in the language five words Gill, three of which have the g hard, while two have it soft: viz. (1) gill of a fish, (2) gill, a ravine, both of which are Norse, and (3) Gill, the surname, which is mostly Gaelic = White; and (4) gill a liquid measure, from O. Fr. gelle, Late Lat. gella in the same sense, and (5) Gill, a girl’s name, shortened from Gillian, Juliana (see Skeat’s Etymological Dictionary). No one of these words is of native origin; otherwise the initial g would have changed to y, as in Eng. yell from the O. Eng. gellan, giellan.

GABBRO, in petrology, a group of plutonic basic rocks, holocrystalline and usually rather coarse-grained, consisting essentially of a basic plagioclase felspar and one or more ferromagnesian minerals (such as augite, hornblende, hypersthene and olivine). The name was given originally in north Italy to certain coarsely crystalline dark green rocks, some of which are true gabbros, while others are serpentines. The gabbros are the plutonic or deep-seated representatives of the dolerites, basalts and diabases (also of some varieties of andesite) with which they agree closely in mineral composition, but not in minute structure. Of their minerals felspar is usually the most abundant, and is principally labradorite and bytownite, though anorthite occurs in some, while oligoclase and orthoclase have been found in others. The felspar is sometimes very clear and fresh, its crystals being for the most part short and broad, with rather irregular or rounded outlines. Albite twinning is very frequent, but in these rocks it is often accompanied by pericline twinning by which the broad or narrow albite plates are cut transversely by many thin, bright and dark bars as seen in polarized light. Equally characteristic of the gabbros is the alteration of the felspars to cloudy, semi-opaque masses of saussurite. These are compact, tough, devoid of cleavage, and have a waxy lustre and usually a greenish-white colour. When this substance can be resolved by the microscope it proves to consist usually of zoisite or epidote, with garnet and albite, but mixed with it are also chlorite, amphibole, serpentine, prehnite, sericite and other minerals. The augite is usually brown, but greenish, violet and colourless varieties may occur. Hypersthene, when present, is often strikingly pleochroic in colours varying from pink to bright green. It weathers readily to platy-pseudomorphs of bastite which are soft and yield low polarization colours. The olivine is colourless in itself, but in most cases is altered to green or yellow serpentine, often with bands of dark magnetite granules along its cleavages and cracks. Hornblende when primary is often brown, and may surround augite or be perthitically intergrown with it; original green hornblende probably occurs also, though it is more frequently secondary. Dark-brown biotite, although by no means an important constituent of these rocks, occurs in many of them. Quartz is rare, but is occasionally seen intergrown with felspar as micropegmatite. Among the accessory minerals may be mentioned apatite, magnetite, ilmenite, picotite and garnet.

A peculiar feature, repeated so constantly in many of the minerals of these rocks as to be almost typical of them, is the occurrence of small black or dark brown enclosures often regularly arranged parallel to certain crystallographic planes. Reflection of light from the surfaces of these minute enclosures produces a shimmering or Schiller. In augite or hypersthene the effect is that the surface of the mineral has a bronzy sub-metallic appearance, and polished plates seen at a definite angle yield a bright coppery-red reflection, but polished sections of the felspars may exhibit a brilliant play of colours, as is well seen in the Labrador spar, which is used as an ornamental or semi-precious stone. In olivine the black enclosures are not thin laminae, but branching growths resembling pieces of moss. The phenomenon is known as “schillerization”; its origin has been much discussed, some holding that it is secondary, while others regard these enclosures as original.

In many gabbros there is a tendency to a centric arrangement of the minerals, the first crystallized forming nuclei around which the others grow. Thus magnetite, apatite and picotite, with olivine, may be enclosed in augite, hornblende, and hypersthene, sometimes with a later growth of biotite, while the felspars occupy the interspaces between the clusters of ferromagnesian minerals. In some cases there are borders around olivine consisting of fibrous hornblende or tremolite and rhombic pyroxene (kelyphitic or ocellar structures); spinels and garnet may occur in this zone, and as it is developed most frequently where olivine is in contact with felspar it may be due to a chemical resorption at a late stage in the solidification of the rock. In some gabbros and norites reaction rims of fibrous hornblende are found around both hypersthene and diallage where these are in contact with felspar. Typical orbicular structure such as characterizes some granites and diorites is rare in the gabbros, though it has been observed in a few instances in Norway, California, &c.

In a very large number of the rocks of this group the plagioclase felspar has crystallized in large measure before the pyroxene, and is enveloped by it in ophitic manner exactly as occurs in the diabases. When these rocks become fine-grained they pass gradually into ophitic diabase and dolerite; only very rarely does olivine enclose 