the umbilical veins returning 'the blood from the placenta; there is, therefore, a continuous line of ascent from the lateral line veins of the fish to the umbilical vein of man. In reptiles, birds, monotremes, marsupials and many rodents, insectivores, bats and .ungulates, a left superior vena cava (precaval vein) is present as well as 5. right; it passes ventral. to the root' of the left lung and then dorsal to the left auricle of the heart until it reaches the coronary sinus to open into the right auricle. Its course is indicated in man by the left superior intercostal vein, the vestigial fold of Marshall (see Coelom and Serous Membranes) and the oblique vein of Marshall. It can be readily reconstructed from figs. 4 and 5 if the transverse communication (L.I.) is obliterated. In some mammals the postcaval vein is double, especially, in its hinder (caudal) part, and this sometimes occurs as a human abnormality (see F. W.. McClure, Am. Journ. of Anat. vol. 2, 1903, and. vol. 5, 1906, also Anat. Anzeiger Bd. 29, 1906). ' ' i-Except
in Cetacea, one or both azygos veins are always present in mammals. When there is only:one it is usually the right, though a. few forms among the marsupials, rodents and ungulates have only the left (F. E. Beddard, P.Z.S., 1907, p. 181). In many of the lower mammals the external jugular vein is much larger than the internal and returns most of the blood from the brain through an opening called the postglenoid foramen. For. this reason it was formerly regarded as the representative of the primitive jugular. It.is now, however, thought that the internal jugular is that representative, and that the arrangement of man, in which the internal jugular drains the interior of the cranium, is the more generalized and primitive.
For further details and literature see R. Wiedersheim's Comparative Anatomy of Vertebrates, translated by W. N. Parker (London, 1907). (F. G. P.)
VEINS, in geology, masses of rock which occupy fissures in
other rocks. They may have originated in many different ways
and present a. great variety of forms and structures. We may
classify them in three groups: (i.) veins of igneous rock, (ii.) of
sedimentary, and (iii.) of minerals deposited by water or by
gases.
Veins of igneous rock are practically the same as dikes; yet a distinction is sometimes made that dlkes .are narrow, often straight-walled and run for considerable distances, while veins are irregular, discontinuous and of limited extent. Where granite invades sedimentary or metamorphic rocks it very commonly emits vast numbers of dikes. The margin of the granite is full of blocks of all sizes, so that it is often impossible to say where the solid granite ends and the fringe of veins begins. An intrusion plexus of this sort seldom extends for more than a few hundred yards; many granites, on the other hand, have sharp and well-defined margins and send few veins into the country rock.
In plutonic rock areas veining is also very common. Great intrusive masses have not as a rule been injected in one stage but have been slowly enlarged by gradual or repeated inflows, and often the earliest portions 'had consolidated before the last were introduced. Very frequently the older rocks are of a different character, being usually more basic than those. which succeed them, and this makes the veining more obvious. For instance, it is common toilind peridotite traversed 'by many veins of gabbro, or diorite injected with numerous veins of granite, though in either case the rocks are part of one plutonic, boss or laccolite. The crystalline structure of the vein-rock and the surrounding mass is usually quite similar and there may be no fine-grained edges to the veins; these facts establish that the older mass though solid had not yet cooled down, so that the veining is directly connected with the injection process and the two rock shave been derived from the same source, but one is slightly later than the other.
Among the Laurentian or Lewisian gneisses, which resemble granites, diorites and gabbros in composition, but have a banded, or foliated structure, veining of this type is almost universal. The veins are of all sizes and of very irregular shape. Frequently they run along the foliation of the gneiss, but often also they cross it obliquely or at right angles. Such gneisses were produced by the injection of a partly differentiated and consequently non-homogeneous magma, by successive stages, under a rock crust which was in movement or was subjected to intermittent pressures during consolidation.
In certain cases the new material introduced into the rock by these veins bulks almost as largely as the original substance. A shale, slate or phyllite is, sometimes so filled with threads of granite that its composition and appearance are completely altered. Thin pale threads of quartz and felspar, not more than a tenth of an inch in thickness maybe seen following the bedding planes, or the cleavage and sometimes also the slip-cleavage. The distance between the veins may be no greater than the breadth of the veins themselves, and' thus a stripe or banded rock is produced, resembling a gniess but of dual origin, a mixed rock which is described properly as a “composite” or “synthetic” gneiss. The French geologists who first insisted on the importance of this group of rocks have called the process lit par lit (bed-by-bed) injection. The best examples of this in Britain are to be found around the granites of Mull and northern Sutherlandshire. The rocks invaded by granite in this manner often show intense contact alteration and are to a large extent recrystallized.
The short irregular veins which commonly occur within areas of granite, diorite, gabbro and other plutonic rocks are often much more coarsely crystalline than the rock around them. This is no doubt partly due tg the high temperature of theivhole complex and to slow crystallization; but it may also be ascribed to the action of vapours dissolved in the magma and gradually released as it solidifies. Such coarse-grained igneous rocks are called pegmatites (q.v.). It is clear that they are not, purely-igneous but are partly pneumatolytic.
With the pegmatites we may class the fine-grained acid veins (aplites) which are found not only in granites but also in many diabases. They occur in irregular streaks orias long branching well-defined veins, and are usually more rich in quartz and felspar than the surrounding rock. Formerly they were often described as contemporaneous, or as segregation veins; but no vein can be in strict accuracy contemporaneous with the rock which it intersects, and many of them give evidence of having been intruded into their present situation, since their minerals are so arranged as to show flexion structure. But they are always intimately connected, as their mineral composition indicates, with the rock mass in which they lie, and they represent merely the last part of the magma to consolidate. The fissures they occupy are presumably due to contraction, seeing that they are not accompanied by displacement, brecciation or faulting.
Veins of sedimentary rock are few and of little importance. They occur where sediment has gathered in cavities of other rocks. Lava streams, for example, when they cool, become split up into irregular blocks, and in the crevices between these ashes, sand and clay will settle. Submarine lavas are often traversed by great numbers of thin, veins of sandstone, and a similar phenomenon may also be noted in the tuff of submarine necks or other ash beds. Cracks in limestone and dolomite are widened by the solvent action of percolating waters and rnay be filled with gravel, soil, clay and sand. In the Carboniferous Limestone, for instance, veins of bedded sandstone sometimes pass down from overlying Triassic deposits. The upper surface of the chalk in the south of England has frequently many deep funnel-shaped, pipes which are occupied by Tertiary or recent accumulations. .
The third group of veins, namely, those which have been filled by deposits from solution in water or in vapours, is of the greatest importance as including a very large number of mineral veins and ore-bodies. They are also the source of the great majority of the finely crystallized .specimens of minerals.
The deposition of minerals on the walls of fissures by a process of sublimation may be observed at any active volcano. The cracks in the upper part of lava flows are often lined by crystals of sal-ammoniac, sodium chloride, ferric chloride and of volatile substances; By oxidation of the iron chloride bright scales of haematite (ferric oxide) arise; sulphurous acid and sulphuretted hydrogen, given out as gases, react on one another, producing yellow encrustations of sulphur; and copper oxide (tenorite) and a great variety of other minerals (alum, iron sulphate, realgar, borates and fluoride) are found about fumaroles of Vesuvius and other volcanoes.
Most veins, however, are not of superficial origin but have been formed at some depth. The heat given out by masses of rocks which were injected in a molten state is no doubt sufficiently high to volatilize many minerals. The pressure, however, also must be taken into account, as it tends to retain these substances in a liquid condition. Water vapour is always the most abundant gas in a volcanic magma, and next to it are carbonic acid, sulfurous acid, sulphuretted hydrogen and hydrochloric acid. The physical condition of the substances passing outwards from an igneous mass through fissures in the superincumbent rocks will depend on the nature of the substances, on the temperature and the pressure. Near the granite the heat is so great, at first at any rate, that gaseous materials must greatly preponderate; but farther away many of them will be condensed and hot aqueous solutions of complex composition will fill the cracks.
Veins deposited by the action of gases and vapours are said to be of “pneumatolytic,” origin; where hot aqueous, solutions have
