diameter, and adapted to carry a porcelain or copper hot-water bath, or a small brass plate. The former is illustrated in section at H, and the latter at I, and their use to the microscopist in the practice of preparing and mounting objects is not easily overrated. The larger ring carries the ordinary reading-shade, and both are concentric with the burner D. Three carriers of the form J are attached radially to the burner D, and upon them the porcelain white cloud-shade K rests, while a blue spot upon the lamp-glass of D corrects the yellow rays (less in number in the Silber burner at any time than in ordinary petroline flames), and renders this, to my mind, the most powerful and most complete microscope lamp known. I shall have the pleasure to introduce the first one manufactured to the notice of the members of the Royal Microscopical Society at their next meeting, and immediately afterwards they will be supplied by the Silber Light Company (Limited) and their agents; but I should perhaps at once say that I have no interest whatever in their manufacture, beyond a desire that the "Sear Lamp" may prove a useful and convenient accessory to the use of my favourite instrument, the microscope. W. LANE SEAR, Hon. Lib. Margate Micro. Club. A CHAPTER IN THE HISTORY OF ROCK STRUCTURE. BY THE REV. J. MAGENS MELLO, M.A., F.G.S., &c. President of the Derbyshire Microscopical Society. THE study of rock structure is one of great in terest to the geologist, and not only does it teach him the various materials of which any particular rock is built up, but it will often lead him to the knowledge of wonderful facts relating to its origin and past history, and will enable him to trace some of the many changes to which it may have been subjected during the lapse of time. homogeneous stone-such, for instance, as is a bit of flint-but that it is built up of various dissimilarlooking materials; and we may notice, moreover, that one or more of those materials is crystalline, that it is shaped in some regular geometrical form. We shall probably be struck with certain whitish or flesh-coloured crystals, more conspicuously promi Fig. 49. Section of granite from Cornwall (polarized), magnified 26 diam. I propose to illustrate this by taking some familiar specimen and showing the ways in which we may investigate its nature and history. Suppose we take a piece of granite and see what we may learn about it. There are few persons but are acquainted with this rock in some one or more of the forms in which it is found. Our public buildings often present us with splendid illustrations of granite, sometimes roughly hewn, as it has come from the quarry; in other cases highly polished. We have seen the fine grey stones from Aberdeen, or the beautiful red ones from Peterhead and elsewhere. Now when we begin to examine a piece of one of these granites, we see at once that it is not a rock is composed; the larger opaque crystals, whether white or pink, are felspar, the glassy mineral is quartz, and the little glittering spangles are mica. We may next proceed to a more detailed examination of each of these in turn. We will first ask the chemist what he can tell us of their composition. The chemist is not satisfied with merely knowing that a certain mineral occurring in certain definite crystalline or other forms is quartz, another felspar, and so on; but he asks further,-What is this quartz? Is it a simple body or is it, simple as it may appear to sight, a compound of two or more elements? He takes various specimens of quartz, some perhaps from the granite, others from some other rocks, and subjects them to the analytical processes of the laboratory: the result is that he finds all quartz, no matter what its colour may be, whether white or pink or black, or pure and colourless as glass, to be a compound of the metalloid silicon and the gas oxygen; in other words, that it is an oxide of silicon, to which he assigns the name silica. By a series of analyses he is able to correlate the quartz of the granite with all other forms, and they are many in which this mineral occurs. The flint of the chalk, the white veins so often met with in the older slaty rocks, the agates picked up on the sea-shore and elsewhere, the beautiful crystals known as cairngorms, amethysts, and others, are all found to be but varying forms of the same substance, coloured sometimes by adventitious matter, as iron, &c.; and he finds, too, that the exquisite skeletons of some of the sponges, the delicate valves of the diatomaces and other minute specimens of organic life, consist of this very same silica, which is indeed one of the most important compounds entering into the structure of the earth's crust. Suppose the student next picks out one of the felspar crystals: this on analysis will be, as was the quartz, found to be also a combination; in it he will also find silica, but the silica in this instance is found to be combined with the metals aluminium and potassium,-in fact, is a double silicate of alumina and potash. There are many varieties of felspar, some of them differing from that most common in granite, which is called orthoclase, in containing lime or soda instead of potash; these are also distinguished from the orthoclastic species by their crystalline structure, which will afford, as we shall see, a ready method for their recognition when they are microscopically examined. When the granite rocks become decomposed, as they often do in Cornwall and elsewhere, through the wear and tear of the weather, we frequently find the disintegrated materials so separated that the silicate of alumina of the felspar forms thick deposits of the beautiful white clay known as Kaolin, and which is so valuable to the china-manufacturer. The mica of granite is usually a variety called Muscovite, or potash mica; this again on chemical analysis is found to contain, as did the felspar, silica, alumina, and potash, and also often some iron and manganese. There are several different sorts of mica, also, sometimes found in granite, especially Biotite, the composition of which varies from the above; but all the micas may be known by their being found in flattish crystals, which may be split up into an infinity of thin leaflets. Thus far our unaided eyesight and the help of the chemist have shown us what granite is made of; but we are now beginning to learn that, would we know something of the real history of a rock, a far minuter examination is needful, and geologists are rapidly learning that they must turn to the microscope if they would receive answers to many important questions, both as to the history and also as to the composition of rocks. A marvellous light has been shed during the past few years on rock-structure through this minute investigation, especially with the aid of polarized light. The intricacies of the closestgrained rocks have been disentangled, their component parts distinguished from each other, and the very order and history of their combination in the mass revealed. Now, when we examine our granite beneath the microscope, which can be done by having thin slices prepared, we shall learn something about it which we could hardly hope to have discovered without this aid. There has been much speculation as to the origin of granite, whether it is a plutonic—that is, an old volcanic rock—or whether it is only a deposit from water consolidated and altered during the lapse of long ages by heat and pressure: the microscope will help us to the truth. When magnified and examined with the polariscope, a thin section of granite is a very beautiful object, and its different constituent parts stand revealed with the greatest distinctness: we at once learn to see the crystals of felspar, somewhat opaque and cloudy as they usually are in granite, but now and then clear and beautifully striped, and also the crystals of mica, embedded in the clear quartz, which will be at once known by its bright clear colours and by the margin of rainbow-like tints which border its patches. Ordinary orthoclase felspar is usually somewhat opaque and dirty-looking under the microscope and by this it may be distinguished from the clear glassy sanidine which is frequently found in igneous rocks, and presents under the microscope, when polarized, pure rich colours as well as sharplydefined crystals similar in form to those of the common orthoclase. The orthoclastic felspars may be very readily distinguished from the plagioclastic by their structure, as revealed by the polariscope; the latter invariably are seen to be striped with variously coloured bands, showing what is called twin crystallization; and the orthoclase, though often forming twins on a larger scale, does not present the minutely banded appearance of the plagioclastic felspars. The mica in the granite section will not be difficult to recognize, especially if Biotite; often we shall observe it as forming fairly-shaped hexagonal crystals, and the polariscope will also help us to know it by its thinly laminated structure, giving rise to fine parallel striæ on the surface of its crystals. Its colours, also, when polarized will be duller than those of the quartz, for which it might sometimes be mistaken at first sight, should it be a lightcoloured mica; and then, again, it will frequently be found that when the prisms of the polariscope are crossed the mica becomes perfectly opaque, its sections having been formed across the optical axis. But let us now look at the quartz. We shall observe that this quartz is generally not crystallized in definite forms, as are the felspar and the mica; it appears as a matrix which has been at some time or condition shows it to belong undoubtedly to the igneous class of rocks, but to have been formed under conditions differing from those which have given rise to lavas reaching the surface. As far as can be gathered, the granite rocks, as such, have never seen the light of day until exposed by denudation, &c.; their origin was deep in the central portions of ancient volcanoes, where, by partial melting and slow cooling, under intense pressure, and in the presence of some water, the various minerals came together and crystallized into granite. NOTES ON THE DIPTERA.-IV. other soft and so is penetrated by the other crystals, SING INCE our space is necessarily limited, we are compelled to pass by many flies which otherwise we should like to describe. As the Helomyzides are a large sub-family, we will take another example from them—namely, Tetanocera. Flies of this and two or three other allied genera are remarkable for their oddity. There are ten or eleven species of Tetanocera tolerably common, so that it is difficult to know which to choose for description. At fig. 55 is drawn the head of Tetanocera marginata, which, although not so common as one or two other species, is perhaps the oddest fly of the whole genus. It is the form of the head and of the antennæ particularly which gives these flies their peculiarity,--indeed, a Tetanocera may always be recognized by its antennæ, which are carried horizontally, and always have the third joint more or less pointed. In T. marginata the second joint is much longer than the third. This is the characteristic feature of the fly. Its general colour is rusty, and its wings are speckled brown and white. Another 'species which we frequently meet with is T. cucullaria. This, too, as are all the common species of Tetanoceræ, is a rust-coloured fly. It is not unlike a dung-fly in shape, but the head, instead of being round, is somewhat flattened, and the face is white. It may be distinguished from other Tetanoceræ as follows:-The bristle of the antennæ is covered with short downy bairs, while its second and third joints, unlike the antennæ of T.marginata are of equal length. The wings are transparent, with a number of indistinct pale brown dots towards the tip. But perhaps the commonest Tetanocera is T. Hieracii, which can generally be procured from marshy places. Its characteristic features are these: the bristle of the antenna is the interspaces of which it fills up: this shows us at once that it must have been solidified after them, and so was unable to assume its regular forms. This is a very remarkable fact, and helps us towards the secret of the formation of the granite. We know that quartz requires a higher temperature to melt it than does either the felspar or the mica, and so, had the granite been formed as are regular volcanic rocks in the ordinary way of igneous fusion, we should certainly have found that the quartz would have crystallized before either the felspar or the mica, and it would have been seen in definite crystalline form, and its crystals would have interfered with and penetrated those of the other mineral constituents of the rock. Again, if we look carefully at the quartz with a moderately high power, we shall see in it certain small cavities, and some of these will be seen to contain a certain amount of liquid, and also an air-bubble, which will move as the specimen is moved. This liquid has been proved to be water, and from the fact of its not entirely filling the cavity we learn that a reduction of tem. perature has taken place since the water was first caught up by the quartz, causing the contents of the cavities to contract. Sometimes we shall find other cavities, which, instead of containing water, contain small crystals, or even air only. Now, from all these facts it appears tolerably certain that the granite was formed under peculiar circumstances; it has never been such a purely molten rock as is the lava of a volcano, which is poured out from its crater to the light of day. We gather that it was rather formed at great depths in the earth, where it may have been partially melted, partially subjected to the action both of water and of steam, charged with various mineral substances, and sub-fringed with hairs, while the third joint is slightly jected to enormous pressure. What the original condition of granite was we cannot tell: some have gone so far as to think that it may have been that of a sedimentary rock, which has been metamorphosed by the forces just alluded to. But whatever the primary state of granite may have been, its present longer than the second. Its wings are dark brown, covered with three transverse bands of transverse spots, and the discal transverse vein undulates slightly, but not so much as in fig. 30. The mouths of the Muscidæ are all formed on one type, which, however it may be varied, can be recognized at a glance. As flies of the genus Tetanocera have mouths of the normal shape, we have selected them for illustration and description. Fig. 53 is a drawing of the mouth of Tetanocera Hieracii magnified 45 diameters, and viewed as a transparent object to show its muscular action, which, as it is difficult to understand, we describe somewhat at length. Since everybody is not versed in anatomical terms, we would premise that a muscle is fixed to the skeleton (or to those portions of the animal which serve as the skeleton) by two spectively the exsertor and retractor muscles of the mouth. The figure shows the pair contracted; consequently the mouth is pushed out from the head. If the muscle c were similarly contracted, the pair b would relax, and the mouth be withdrawn. At a are the pharyngeal muscles. They arise from the top of the pharynx, and are inserted into the plater, fig. 54. (Shown in fig. 53, but not lettered.) It forms the roof of the mouth, and it is joined to the sides of the pharynx. It has a certain limited up-and-down motion, and it is shown nearly shut in fig. 53, and wide open in fig. 54, as if for the Fig. 53. Mouth of Tetanocera, x45 diams., as a transparent object: ph, ph, Pharynx; lbr, Labrum or upper lip; f, one of a pair of fulcra which move it; /, Lingua or tongue; la, Labium or lower lip; mt, Mentum; f", Processes which terminate it; d, d", edge or rim of the oral cleft; e, Esophagus; sd, Salivary duct; v, Valve in the same. Names of the muscles, and their points of origin and insertion:-b, Exsertors of the mouth, arising from the rim of the oral cleft at d, and attached to the processes of the pharynx; e, Retractor of the mouth, from d" to mt; a, Pharyngeal muscles, from top of pharynx to the roof of the mouth; e, Depressor of the labium, from tip of pharynx to e; g, Elevator of the labrum, from apex of the pharynx to end of the fulcrum f; mt, Muscle of the mentum, from its base to its tip f"; k, Transverse muscle of the mentum, from e to k. points only, at one of which it is said to "arise," while at the other it is "inserted"; and further, a muscle pulls its point of "insertion or "attachment" towards the point where it has its origin, i.e. "arises." The mouth of a fly has three joints, and the part which first attracts our attention is the pharynx, which forms the basal joint (ph, fig. 53, and shown in section at fig. 54). A good idea of its shape may be obtained by cutting an isosceles triangle out of a piece of paper, folding it down the middle, and fastening together the two corners which possess the equal angles. Our model pharynx, however, will not possess the two posterior "processes," nor the long and narrow chitonous plate which moves up and down between its sides. To the processes are attached a pair of muscles, b, which arise from the rim of the oral cleft at the point d; at d", on the opposite side of the rim, arises a muscle c, which is inserted at the base of the mentum. These are re passage of food. The raising and lowering of this plater produces suction, and it is moved, of course, by the muscles marked a in both figures. At lbr is the upper lip or labrum, which when at rest fits tight over the labium. It consists of an inner and an outer plate connected by transverse muscles (see figure). The inner plate is continuous with the plate r, as the roof of the mouth. At f is shown one of the fulcra which move the labrum. There are a pair of them, one on each side, and they are articulated to the base of the outer plate. They are the homologues of the maxillæ in Syrphide and other flies. At the top of the pharynx arises a pair of long muscles, g, inserted where the fulcra join the labrum. These muscles elevate that organ, as shown in the drawing: attached to the other end of the fulcra are the muscles h, which depress it; while another muscle (only faintly drawn), which has its origin at the tip of the pharynx, aids g in raising it. These three pairs of Fig. 54. Mouth of Tetanocera; diagram illustrating the action of the pharynx, x 45 diam.; ph, Pharynx; c, Its tip, where numerous muscles are attached; br, Commencement of labrum: a, Great pharyngeal muscle; b, small muscle; a, Esophagus; r, Roof of the mouth, which moves up and down. Fig. 55. Head of Tetanoceras marginata, × 14 diam. Fig. 56. Head of Sepedon sphegeus, × 14 diam. is the injection of saliva into the wound made by the labrum that produces irritation and swelling. The whole of the second and third joints of the mouth are called the labium. This is of three parts -an outer plate (mt, fig. 53), which is the mentum ; an inner plate, e, which forms the floor of the mouth, and is the labium proper; and, thirdly, a pair of lobes. The mentum terminates in a pair of long processes (f", fig. 53), which are attached to the outer skin of the lobes by means of a triangular thickening of the Fig. 57. Head of Loxocera ichneumonea, 12 diam. is attached a muscle arising from the tip of the pharynx, which bends down the labium when the fly wishes to bring its lancets into action. The inner surfaces of the lobes are traversed by about thirty minute gutters, generally known as "false tracheæ," but which, since that name would lead one to suppose that they are closed tubes instead of open gutters, we prefer to call "capillary channels." The food of the fly, which is almost entirely liquid, is collected by these, and passes between the labrum and labium into the pharynx, which, as before noted, supplies the power of suction. The whole mouth is liberally supplied with trachea, which ramify so much that even each of the hairs on the exterior surfaces of the lobes has a separate branch. We hope that this figure and description will enable amateur entomologists to understand how a fly eats, which is somewhat difficult to make out from specimens prepared in the ordinary way. Fig. 56 is an outline of the head of Sepedon sphegeus, a fly belonging to the same sub-family as Tetanocera, and in many respects resembling that genus. It is not common enough to be described here, but the drawing is given to show how widely the antennæ differ from the ordinary type prevailing among the Muscidæ, and approach the type found in the Conopida. Passing over several sub-families, we come to the Psilides (i.e. "smooth flies "), to which belong two insects better known than liked; viz. the Cheese-fly (Piophila casei), and the Bacon-fly (P. luteata). These are the parents of the "hoppers" which infest cheese and bacon. Belonging to this sub-family there is a fly of very paradoxical appearance, named Loxocera ichneuThe name Loxocera (meaning "oblique horn") is given to the genus because the antennæ, monea. |