of the rock; but sometimes these accessory ingredients impart a special character to it, and so to a certain extent pass into essentials. Their presence creates the varieties of species.

The constituent minerals (whether accessory or essential) of any given rock either occur in separate crystals or particles distinguishable by the naked eye, or they consist of smali finely divided particles so intimately blended together as apparently to form a homogeneous mass; nevertheless, in the latter case, their separate existence may be generally recognised by magnifying power.

The first and principal requisite for the student of Lithology is to be able to recognise and determine the minerals of which a given rock consists. This is in many cases no easy task; he must therefore have a competent knowledge of mineralogy. Not with a view adequately to supply the want of such knowledge, but by way of introduction to our subject, and for the purpose of reference in the absence of more comprehensive works, we propose to give in this chapter a brief notice of the principal minerals with which we have to do in examining the structure of rocks, adding such particulars as are more especially useful for our present purpose.

The number of these principal minerals is relatively very small. They may be classed under the following comprehensive names:- FELSPAR, QUARTZ, MICA, HORNBLENDE (Amphibole), PYROXENE (Augite), CALCSPAR, and DOLOMITE. The following occur less frequently :-CHLORITE, TALC, LEUCITE, NEPHELINE, OLIVINE, TOURMALINE, GARNET, GYPSUM, COAL, some SULPHURETS, and some IRON ORES.

The number of the accessory ingredients is very much greater, and indeed almost unlimited; that is to say, under certain circumstances almost every known mineral may occur as an accessory in any rock, and the essential ingredients of one rock frequently occur as accessories in another rock. But although we may say with truth that the number of the accessory minerals is without limit, yet in fact only about a quarter of the number of hitherto known minerals occur in rocks so abundantly and frequently as to be specially noticed in a treatise of Lithology. One consideration is particularly deserving the atten

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tion of the scientific observer of rocks; we refer to what is termed by Breithaupt the Paragenesis' of minerals. By this is meant the law of mutual association or repulsion of certain minerals. It is well known to mineralogists that the presence of one mineral very frequently denotes the neighbourhood of another, and, vice versâ, that the presence of some minerals forbids the simultaneous presence of certain others.

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:

In 1849 Breithaupt first treated this subject, and published in his Paragenesis der Mineralien' a great number of remarkable instances of this law. We may, for the sake of illustration only, select the following as examples: 1. Minerals which are usually associated together:Quartz and mica; orthoclase, quartz, and mica; orthoclase and oligoclase; labradorite and augite; orthoclase or oligoclase and hornblende; hornblende and epidote. 2. On the other hand, quartz and augite appear each to exclude the presence of each other; also (according to Roth) labradorite and hornblende (?).

We are unable to pursue this important subject in this place; we have been compelled to confine ourselves, in the following notice, to appending a few of the more important instances of paragenesis to the description of some of the principal mineral classes.

As to the much-debated question of classification of the minerals, we have adopted one which appeared to us best suited for our present purpose; it is not exactly that of any one author. We have placed a few of those minerals first which are of the most frequent occurrence; otherwise, the arrangement adopted will be found to correspond in several respects with Dana's System of Mineralogy.'

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The following are the abbreviations we have used :H. for hardness; S.G. for specific gravity; Cp. for chemical composition; Bp. for before the blowpipe. The quantities of the chemical elements we have given in round numbers, as being sufficient for our present purpose. In the chemical formulæ we have, for the sake of convenience, adopted the abbreviations usual on the Continent, of expressing the oxygen atoms by dots, and a stroke to denote a double atom; thus, Fe2O3 is written Fe. We subjoin the following list of formulæ for the elementary bodies and their simple compounds:

1. Oxygen Compounds.

A. OXIDES OF SILICON AND ALUMINUM (EARTHS). 1. Quartz.-Rhombohedral crystals, usually combinations of two rhombohedrons and hexagonal prism. Cleavage according to the planes of one rhombohedron, but imperfect. Fracture conchoidal to uneven and splintery. H.=7. S.G.=2.5— 2.8. Colourless and limpid, or variously coloured, forming many varieties. Lustre vitreous, sometimes resinous, especially on the surfaces of fracture. Cp. Si, with admixture of minute particles of colouring oxides. Two modifications of chemical composition are distinguished by their different degrees of solubility. The one is insoluble in water and in every acid, except hydrofluoric acid; the other is soluble in water at high temperatures, especially in the presence of other acids and alkalies. The insoluble variety of quartz may, in process of time, become converted into the soluble by the contact-influence of infiltrated moisture. The soluble variety of quartz, in small proportions, is found in many waters of springs and rivers, and in the sea, e. g., at the Geysers in Iceland, up to 1800 per cent. and in sea-water to 100000 per cent. Bp. infusible; with soda fusible to a clear glass with effervescence. affected by phosphoric acid.

(a) Common Quartz, the most abundant of all minerals.
It is found:-
:-

(a) As an independent rock. (See post.)

Not

(3) As essential ingredient of many crystalline rocks, especially the plutonic. In most kinds of granite, in greisen, and in the crystalline schists it is found in crystalline grains. In quartz-porphyry, rhyolite, and, exceptionally, in some kinds of granite (e.g. St. Austell, Cornwall), it is perfectly crystallised.

(7) As accessory constituent mass of some rocks (such as

crystalline schists), in form of veins and swellings, or clothing the interior of geodes in other rocks (e.g. in the granites of Switzerland, Carrara marble, the variegated sandstone of the Schwarzwald, &c.) The quartz of the geodes is frequently in the form of transparent crystals (rock crystal), or in greyishbrown to black crystals (smoky quartz, false topaz). (8) As principal ingredient of many fragmental rocks (sandstones, conglomerates). As sand and gravel in beds of deposit.

(b) Amethyst.-Violet, coloured by the oxide of manganese. (c) Chalcedony.-An intimate admixture of crystalline and amorphous silica.

(d) Agate.-A variegated combination of common quartz, amethyst, jasper, carnelian, and other varieties of quartz, arranged in alternate stripes or layers, or irregularly mixed together.

[b, c, and d chiefly occur in the geodes of volcanic rocks (in Iceland, Faroe Islands, the Brazils, &c.), or in metallic veins (e.g. in Saxony).]

(e) Jasper.-Very frequently in globular masses (ball-jasper)

coloured red by the peroxide of iron; found in the bog iron-ore of Briesgau, in Germany, and elsewhere, or coloured yellowish-brown by the hydrated oxide of iron. (Occurs in form of pebbles, e.g. in the sand of the Nile and Desert.) Jasper sometimes forms subordinate layers in other rocks.

(f) Flint.-Coloured greyish-blue, or black, by presence of carbon. Occurs as a concretionary formation in sedimentary limestone rocks, e.g. in the Chalk of England and France, in the Upper White Jurassic of the Franconian Switzerland in Bavaria.