consolidation. For this interesting structure, which one is tempted to call "granulitic," we shall here use the somewhat vague term "granular," or microgranular when necessary Commonly, indeed, the microscope is required for its correct appreciation (see fig. 28); but the absence of well-developed forms, such as prisms of felspar or pyroxene, is noticeable with a lens on the surface of the rock itself.

Ophitic Structure.-Often with the eye the crystals of one constituent will be seen to have developed freely, while another constituent has settled down in large crystals round them, so that the interspaces of the former are filled over considerable areas by material having parallel cleavage-surfaces or crystalfaces. On turning the rock-specimen in the hand, the light will glance from some such surface and show the real continuity of areas that appear distinct from one another on the broken surface of the rock. This structure derives its name from its occurrence in the dolerites and gabbros of the Pyrenees, which were called "ophites" by De Palassou. It is, however, extremely common in the dolerites and diabases of all countries. Prof. G. H. Williams (Journ. of Geol., vol. i., p. 176) has used the term "poikilitic" for what seems practically the same structure, applying it particularly to cases where the enclosing or "ophitic" mineral is felspar. The appearance known as "lustre-mottling" arises when the included crystals are small in proportion to the cleavage-surfaces of the surrounding and subsequently-developed mineral. "Lustre-mottling" is common in the Peridotites. (See fig. 39, and Index.)

Pegmatitic or Graphic Structure. Two constituents, most commonly quartz and felspar, have developed simultaneously in large crystals mutually intergrown. The felspar being predominant, the quartz appears as hook-shaped and irregular forms apparently disconnected from one another. The cleavagesurfaces of the felspar thus give the effect of "lustre-mottling;' but the quartz, when examined microscopically, is found also to be optically continuous over considerable areas of the rock. The structure thus resembles that which would be produced if two sponges were to grow up simultaneously, the one filling all the hollows and ramifying passages left by the mode of growth of the other. Graphic granite provides the best and almost only type. (See fig. 25.)

The same structure when minute is styled micropegmatitic. Micropegmatitic intergrowths are often grouped in delicate globular forms around porphyritic crystals of quartz or felspar. These micropegmatitic structures commonly require the micro

scope for their detection, and have been appropriately styled "micrographic" by Harker, and also "granophyric" by Rosenbusch. The term "granophyre" was, however, used by its inventor, Vogelsang, in a sense that included all microcrystalline igneous rocks.

Orbicular Structure. A rare structure in which the crystals are grouped so as to form spheroidal aggregates, with or without radial or concentric arrangement. A fine example is the orbicular diorite ("Corsite") of Corsica. This structure may be regarded, with Vogelsang, as the highest development of the spherulitic. Fluidal Gneissic Structure.-The banded or foliated structure of many holocrystalline rocks arises in some cases during their original flow, and may be designated as above, to distinguish it from the metamorphic gneissic structure. The smaller constituents flow round "eyes" formed by the larger ones; and sometimes the intrusion of a non-homogeneous magma produces a banded gneissic structure on a handsome scale (see, for instance, Geikie, Anc. Volcanoes of Great Britain, vol. ii., figs. 336 and 337). In many other cases, a granitoid rock intrudes in thin sheets along the bedding-planes of a shale, or along the foliation-planes of a schist, and its fluidal gneissic structure is due to the lamina of foreign matter carried off by it.

CHAPTER XII.

SOME PHYSICAL CHARACTERS OF ROCKS.

I. SPECIFIC GRAVITY.-AS will be seen when various rock-types are examined in detail, the specific gravity is often a good guide to chemical constitution. The specimen must be selected with the following precautions:

1. It must be representative of the mass under examination, and sufficiently large to include all the constituents in their correct average proportions.

2. It must be free from flaws and cavities.

3. It must be unweathered, except in certain special investigations.

The general methods of determining specific gravity are detailed upon pp. 22 to 27.

To observe the first precaution, it is often necessary, and, indeed, safer, to use Walker's rather than the refined chemical balance, which will not weigh a specimen of more than 100 grammes. The method devised by Mohr for measuring the displaced water is highly satisfactory in dealing with crystalline rocks of coarse grain and any specimen which it is inadvisable to reduce in size. The displacement-apparatus consists in simple form of an inverted glass bell-jar furnished below with an india-rubber tube and clip and supported on a stand. The water placed in the vessel can be thus run off from below, accuracy being ensured by using the clip rather than a tap, and by letting the tube terminate in a jet formed of glass tubing. A horizontal wooden bar bearing a needle is laid across the top of the vessel, the needle projecting about 3 or 4 cm. downwards. To ensure constancy of position, the points where the bar habitually rests on the glass rim should be marked with a file or by gummed slips of paper.

The vessel is filled with water; the end of the needle is lightly greased, and allowed to project into the liquid. Looking up from below at the bright totally reflecting surface of the water, the clip is released, and the water is allowed to run off until the needle-point just disappears from view. It now exactly touches the upper surface of the water and gives us a standard to which to refer. The specimen, which has been weighed upon a strong but accurate balance, is then lowered by a fine thread or wire

into the vessel, the water rising higher by the addition of its bulk. When all bubbles have disappeared, a graduated measuring-glass is taken, the divisions of which correspond to the units of weight used in the determination of the weight in air. Thus, if grammes were used, the glass will be graduated in cubic centimetres. Into this glass the water is run off until the needlepoint, observed from below as before, again exactly touches the surface of the water. The amount run off gives the bulk of water (d) displaced.

To observe the second precaution, some rocks, such as porous sediments or pumiceous lavas, must be reduced to a powder and determined with the specific gravity bottle, the finest dust being sifted or blown off to avoid choking of the small tube in the stopper.

To observe the third precaution, it is often well to pick up clean chips from specimens trimmed in the field, which, selected from a large number, will serve both for the determination of specific gravity and the making of microscopic sections.

Since the range of specific gravity in rocks, the coals being omitted, rarely exceeds the limits 2-2 to 3-4, many very diverse rocks have the same specific gravity, and the results are not of value in absolute determination. But in the case of igneous rocks, provided that specimens are selected and examined from different parts of an exposure, an excellent idea can be formed, from the specific gravity alone, of the silica-percentage of the

mass.

II. FUSIBILITY.-Though it is seldom desirable, on account of their complexity, to treat rocks before the blowpipe as if they were simple minerals, yet in a few cases the determination of the fusibility proves of service. The older writers relied, indeed, more upon this character than has since been thought desirable, and the nature of the glasses produced was closely studied. It is obvious that the application of the flame, in the absence of an acid, will decide between a soft rock composed of silicates and a limestone, the former in all probability fusing to a glass while the latter becomes luminous and crumbling. The natural glasses also have various degrees of fusibility, the more highly silicated fusing with greater difficulty than the basic. Thus obsidian fuses at about 5 of von Kobell's scale, and tachylyte as easily as 2.5. Care must be exercised, however, in dealing with these glasses that the splinters used do not present unusually thin edges.

The interesting observations of Berger and of Beudant showed that the treatment of volcanic glass in the flame of a blowpipe occasionally results in the formation of a pumice as fusion gradually takes place. (Trans. Geol. Soc., 1816, p. 191, and Voyage en Hongrie, 1822, vol. iii., p. 362.)

The volatile materials thus liberated swell up the whole glass, until in some cases it almost rivals the intumescence of a borax bead. Professor Judd* found that the obsidian granules (Marekanite) of Marekanka, în Siberia, may be converted into a pumice of eight or ten times their original bulk; and similar results are obtainable with the lavas of Krakatoa. The experiment should be repeated, by way of comparison, on any specimen of volcanic glass.

In the case of an igneous rock that has undergone alteration, the fusibility can be of little service, since a very small admixture of hydrous minerals such as zeolites may suffice to considerably increase the fusibility of the mass.

III. HARDNESS. This important property, rendering the use of the knite imperative at all times, has been already referred to on p. 93.

CHAPTER XIIL

THE CHEMICAL EXAMINATION OF ROCKS.

A NUMBER of ordinary qualitative tests may be applied to rocks, and the examination with acids, hot or cold, is naturally of great value in the detection of carbonates.

Pure dolomites, such as at times occur among crystalline masses, will effervesce only when the acid is heated; but magnesia occurs in many limestones in which the acid test is unavailing. The ordinary dolomitic limestones thus etfervesce freely in cold acid, and the magnesia can only be safely determined by precipitation from solution by hydric disodic phosphate in the ordinary way. On the other hand, we must here repeat the warning that a rock which gives no effervescence when touched with strong cold acid may yet belong to the group

*"On Marekanite," Geol. Mag., 1886, p. 243; also, "The Natural History of Lavas," ibid., 1888, p. 6.