LESSONS IN GEOLOGY.-No. XLVII. BY THOS. W. JENKYN, D.D., F.R.G.S., F.G.S., &c. CHAPTER V. ON THE CLASSIFICATION OF THE ROCKS IN THE EARTH'S CRUST. SECTION I. A TABULAR VIEW OF ROCKS IN THE VERTICAL ORDER IN WHICH THEY OCCUR. 7 The Maestricht Beds. 8. Upper White Chalk. 9. Lower Chalk II. THE TERTIARY SYSTEM OF ROCKS. The boulder formation or northern drift. Limestone of Girgenti, in Sicily. Sands, clays, and gravels, consisting of fragments of The red crag, and the coralline crag of Suffolk, con- The Sub-Appenine rocks in Italy. The Faluns of Touraine. Some of the beds at Bourdeaux, in France. Part of the molasse of Switzerland. The upper marine beds of Paris Basin, and the sand stones of Fontainebleau. The millstone rocks of the same place. The tile-clays, near Berlin. The tertiary beds about Mayence. The gypsum of Paris. Fresh water limestone, and beds of clays and sands, The Barton beds. The Calcaire Grossier of Paris. Sands, sandstones, gravel of flint pebbles, with beds of Sables inferieurs of Paris. Mottled and plastic clays, with flint pebbles. Coast of Norfolk. At the bottoms of every valley, on the VOL. IV. 90 Loose sand, with bright green particles, and sandstone Warminster; Devizes; Wantage; Shafts. with particles of iron. Freestone of Merstham. Marly stone, with layers of chert. Dark-blue clay, or marl, with small concretions of stone, and many fossils. Sands with green particles, and sandstones with beds of chert. Sands white, yellowish, and ferruginous or irony, with concretions of limestone. A limestone, called the Kentish rag. II. THE WEalden. Clays, with occasional bands of limestone. Sands, with calciferous or limy grits and clays. Limestones, and limy flags or slates, and beds of mari. § III. THE OOLITE. bury. Merstham, in Surrey; Kent. Folkstone; Maidstone; Isle of Wight; Black Down, Devon, &c. Atherfield, Isle of Wight. Maidstone, in Kent. Wealds of Kent, Sussex, and Surrey. Portland stone, a gritty limestone, with beds and nodules Isle of Portland, Swindon, Aylesbury. of chert. Portland sands. Kimmeridge clay, a blue shaly clay, with nodules of septaria or cement stone. Coral rag, imperfect limestone, or a limy freestone, abqunding with shells and fossil corals. Calcareous grit, a silicious and shelly sandstone. Oxford clay, a blue and yellow clay, with Melbury marble, turtle stone, or septaria. Kelloway rock, a coarse and sandy limestone, with many fossils. Cornbrash, an imperfect limestore, sometimes blue and sandy. Forest marble, a coarse, slaty limestone, full of shells. Bradford clay, a tenacious, brown clay, sometimes shaly, full of shells and corals. Great oolite, a yellow freestone, with fragments of shells. The Bath stone. Stonesfield slate, a kind of slate partly limy, partly flinty, passing sometimes into sand with shale." Fuller's earth, a brown clay. Inferior oolite, a coarse, limy freestone, and yellow sands and marl. IV. THE LIAS. White lias limestone, often blue. A blue slaty marl and clay. The limestone full of fossil bones of reptiles. SV. THE TRIAS. The keuper of Germany; variegated marbles, red, gray, blue, green; white sandstones with gypsum. The bone bed of Axmouth. A limestone, compact and grayish, sometimes called muschelkalk, with beds of dolomite and gypsum, wanting in England. Bunter sandstein, or variegated sandstone of Germany. The sandstone spotted red and white, with gypsum and rock salt. Kimmeridge, in Wiltshire; Shotover-hill, Abingdon; Weymouth. Kelloway near Chippenham, Wiltshire, Hinton, near Bath; Frome. Bath; Farley Downs; Combe Downs; Lyme Regis; Whitby; Vale of Bath Axmouth, Dorset, Part of the new red sandstone, and the rock salt beds; Cheshire Worcestershire, IV. THE PRIMARY SYSTEM OF ROCKS. N.B.-Not the Primitive Rocks. § I. THE PERMIAN. Yellow magnesian limestone, sub-crystalline. The zechstein of Thuringia. 21. Upper Permian. Marl slate. 22. Lower Permian. Nottingham; Mansfield; Knaresborough; Lower new red sandstone of the north of England, and Durham; Warwickshire; Staffordshire, the Rothliegendes of Germany Names of Groups according to LYELL. 23. Coal Measures. 24. Upper Devonian. 25. Lower Devonian. 28. Upper Silurian. 27. Lower Silurian, 28. Gambrian Rocks. 29. Chlorite Schist, 30. Mica Schist. 31. Granite. 32. Trap Rocks. A rock composed of crystalline grains of quartz, feldspar, Aberdeen; Dartmoor; Land's End. Unstratified and crystalline rocks, which, in a molten The following diagram will assist you in learning the position and the order of different formations, from the surface soil down to the crystalline and granite rocks, which are supposed to be below the strata marked v in the diagram. Fig. 103. An Ideal Section of the Stratified Rocks, in their vertical order of position. A. Alluvial Soil. B. Erratic Blocks, and Till. D. The Pleiocene. E. The Meiocene. F. The Eocene. G. The Chalk Formation. H. The Quadersandstein. 1. The Neocomien. J. The Wealden. K. The Oolite. L. The Lias. M. The Keuper. N. The Muschelkalk. o. The Buntersandstein. Q. New Red Sandstone, or R. The Coal Measures. s. The Mountain Limestone. The Arcometer of Baumé.--This areometer, which was invented by M. Baumé, of Paris, is one of those having a constant weight, and is very extensively used. It consists of a glass bulb full of air having a graduated stem, with a smaller bulb below it full of mercury to ballast the apparatus when floating in a liquid, fig. 39. This instrument is differently graduated Fig. 39. according as it is intended for liquids denser than water, or for liquids lighter than water. In the former case the weight is so regulated that in distilled water, at the maximum density, it sinks nearly to the upper extremity of the stem, and is in equilibrium at a point marked zero. In order to graduate the stem, fill a vessel with a solution consisting of 85 parts of water by weight, and 15 parts of sea salt. This solution being denser than pure water, the instrument will sink in it only as far as the point B, which is then marked 15. Next, dividing the interval between the points A and B into fifteen equal parts, and continuing the divisions to the bottom of the stem, the instrument is graduated. The divisions are marked on a small slip of paper placed in the interior of the glass stem. The areometer thus constructed can be employed only for T. The Devonian, or Old liquids denser than water, such as acids and saline solutions, Red Sandstone. U. The Silurian. v. The Cambrian. ON PHYSICS OR NATURAL PHILOSOPHY. No. XII. AREOMETERS OF VARIABLE VOLUME. Different kinds of Areometers.-The areometers of Nicholson and Fahrenheit, described in our last lesson, may be defined as those which have a constant volume and a variable weight, because they are always immersed to the same depth in the liquid, and weights are placed on their scale or cup, according to the weight of the solid or the liquid whose specific weight is to be determined. Areometers are also constructed having a variable volume and a constant weight; that is, having no fixed point of immersion level on the stem, and preserving always the same weight. These apparatus, known under the names of hydrometers, scale-areometers, or liquor-tests, are not intended to ascertain the specific weights of liquids, but to determine the strength of saline solutions, acids, and alcohols. being both an acid-test and a salt-test. For liquids not so dense as water, the zero being placed at the bottom of the rod, the graduation is reversed. Baumé determined the zero point of the instrument by its immersion in a solution of 90 parts of water by weight with 10 parts of sea-salt, the point marked 10 on the scale being that which indicated its depth in distilled water. Dividing then the interval between these two points into 10 equal parts, and continuing the divisions to the top of the stem, the instrument is graduated, and becomes a liquortest. These areometers being graduated in a manner entirely arbitrary, indicate neither the densities of the liquids, nor the quantities of salt held in solution. Yet they are usefully employed in ascertaining when a saline or acid solution has been brought to a point of fixed concentration, or a certain degree of strength. The graduation of these instruments assists much in the rapid formation of mixtures and solutions in given proportions, not with very great precision, but with a sufficient approximation in a great number of practical cases. For example, in the manufacture of common syrups, it has been found that the salt-test of Baumé should stand at the mark 35 on the scale as the point of level, in a syrup of proper strength when cool. Thus the manufacturer is furnished with an instrument with which he can readily test the degree of concentration in his syrups. In like manner, in sea-water at the temperature of 82° F., the hydrometer of Baumé stands at the mark 3 on the scale, indicating that the water is of that degree of strength proper for saline baths ordered to patients in certain diseases. The solutions of seasalt and water, which physicians prescribe, are in general much weaker than that indicated by the proper degree on the instrument; that is, the artificial saline baths have not that degree of saltness which the natural sea-water has, and are not therefore sufficiently efficacious in producing a cure. The alcohol-test or measure, invented by M. Gay-Lussac, is | If, therefore, we represent the volume by 100, the volume exactly similar in form to the areometer of Baumé; it differs will be 75. We then mark respectively at the points A and B only in the mode of graduation, this being such that the the numbers 100 and 75. The volume of a B being, according instrument indicates not only the strength of an alcoholic to the value of v', the fourth part of, we divide the space A B mixture, but it also shows how much per cent. it contains of into 25 equal parts, and each of these parts is of AB or TOO water, and how much per cent. of absolute alcohol, that is, of of v, that is, of the volume iminersed in pure water. We next alcohol at its maximum strength. It is graduated in the fol- continue the division to the lower part of the stem, on the lowing manner: The instrument is first immersed in absolute supposition that it is constructed so as to be of exactly the alcohol, and the point or level at which it stands is marked same diameter throughout, that is, wherever a horizontal 100, care being taken to ballast it so that this point is always section may be taken." found near the top of the stem. Mixtures are then formed The instrument being now graduated, suppose that the containing 100 parts in volume of 95, 90, 85, 80, &c., of absolute density of another liquid, say that of sulphuric acid, is required; alcohol, and 5, 10, 15, 20, &c., respectively of water. The immerse the instrument in the liquid, and if it sinks to the instrument is successively immersed in these mixtures, and level or point marked 54 on the stem, this indicates that the the points or levels at which it stands are respectively marked volume of the liquid displaced is represented by 54, that of the 95, 90, 85, 80, &c., accordingly. In order to complete the graduation, it is necessary only to divide each interval into 5 volume of water being represented by 100. Now, as every floating body displaces a weight of the liquid in which it is equal parts. immersed equal to its own, it follows that the volume of water or 100, and the volume of sulphuric acid 54, have the same weight; but the volumes of bodies of equal weights are in the inverse ratio of their densities. Consequently, if we represent the density of sulphuric acid by x, that of water being unity, we have : 1 :: 100: 54; whence x=1.85, which is the density of sulphuric acid. If the instrument thus graduated should sink, for example, to 58 in an alcoholic mixture, this would indicate that in 100 parts of volume, it contains 58 parts of absolute alcohol and 42 parts of water. It is, moreover, necessary to take the temperature into account; for when this increases or diminishes, the density of the alcohol conversely diminishes or increases accordingly, and the instrument consequently sinks more or less in the same alcoholic mixture. To meet this case, GayLussac constructed for his alcohol-test-tables of correction, by means of which the indications of the instrument may be rectified, according to the temperature of the mixture as shown by the thermometer. Saline-tests or measures are also graduated, on the principle of the preceding instrument, to show the quantity of salt by weight contained in different solutions. The zero of these instruments answers to pure water, and they are graduated by dissolving 5, 10, 15, 20, &c., equal parts by weight of a given salt in 95, 90, 85, 80, &c., equal parts by weight respectively of pure water, taking care that in the different solutions the salt and the water are thoroughly mixed. Immersing the instrument successively in these solutions, and marking the numbers 5, 10, 15, 20, &c., respectively at the points where the instrument stands in equilibrium, and dividing the intervals into 5 equal parts, the apparatus is completed. Such instruments have this inconvenience, that every separate kind of salt requires a special saline-test. That, for instance, which has been graduated for the nitrate of potassa, would give indications entirely wrong in a solution of carbonate of potassa. On the same principle are constructed milk-tests, winetests, and spirit-tests, all called by the general name of hydrometers (from the Greek, and signifying water-measures); these instruments are employed in determining the quantity of water which may have been introduced into these liquids for the purposes of fraud. But such instruments are not to be wholly depended upon, since the densities of milk and of wine, for example, are very variable, even when they are in a perfectly natural state; hence, fraud might be attributed to indications which were due rather to the naturally bad qualities of these liquids. Similar test instruments are used by medical men for the liquids found in the human body. Instruments called densimeters (a Latin-Greek compound, signifying density measure) have been invented for the purpose of showing the relative density of a liquid according to the degree to which they sink therein. The densimeter of GayLussac is exactly similar to the areometer of Baumé, represented in fig. 39. It only differs from it in the principle of its graduation, which varies according as it is intended to be used for liquids more or less dense than water. In the former of these cases, the instrument is ballasted, when immersed in pure water, so that it shall sink to the point a at the top of the stem. Taking a liquid of which the density is known, and greater than that of water, say in the ratio of 4 to 3, we immerse the instrument in it, and find that it stands at the level of the point в on the stem. Now, if we represent by v and v' the volumes of the parts of the instrument respectively immersed in water and in the given liquid, these volumes are to one another in the inverse ratio of the densities of these liquids, according to a former lesson: we have therefore v::: 4:3; whence, v'=v. If the densimeter is intended to measure the density of liquids lighter than water, the instrument must be ballasted so that the point marked 100, corresponding to pure water, may be placed at the bottom of the stem. At its upper extremity is then placed a weight equal to the fourth of that of the instrusented by 100, its weight will then be represented by 125. ment. Now, the weight of the instrument alone being repreThis number 125 being marked on the stem, as another point of level, we divide the interval between the points marked 100 and 125 into 25 equal parts, and continue the divisions to the top of the stem. The application of the densimeter of Gay-Lussac requires a quantity of liquid sufficient to fill a vessel of considerable capacity. In certain cases, however, especially in physiology, when experimenting on animal liquids, it often happens that we can only obtain a few grains of the animal matter. This led to the invention of the densimeter of M. Rousseau, which accomplishes the object in view. This instrument is of the form of the areometer of Baumé, fig. 40; but the top of the stem is Fig. 40. furnished with a small cup for the reception of the liquid whose density is required. We shall here show how the inventor graduated his instrument, according to the French system of weights. On the sides of the cup is placed a mark indicating a capacity a c, equal to that of a cubic centimetre (or 06103 of a cubic inch, which is rather less than of a cubic inch). In order to graduate the instrument, it is ballasted in such a manner that in distilled water at the maximum density it sinks to the point в at the bottom of the stem, and this is the zero point of the instrument. The cup is then filled with |