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mostly herbaceous, some of which are found in almost all parts of the world. Perhaps the most important of them is the foxglove or digitalis, which is used in medicine. Many of them have flowers of considerable beauty, as the calceolarias, veronicas or speedwells, and antirrhinums or snapdragons. The flowers of the Scrophulariaceae have very often more or less that peculiar form which is seen in the calceolarias and antirrhinums. Another large order is that of Labiata, from Latin labium, a lip, in which also the corolla generally divides into two parts or lips. The plants of this order are mostly herbaceous plants or small shrubs, and many of them are remarkable for their fragrance, which is due to an essential oil abounding in their leaves. Some of them are used in cookery for flavouring, some in perfumery, and some in medicine. As examples of this order, it is enough to name mint, thyme, lavender, patchouli, sage, hyssop, rosemary, marjoram, basil, and savory.-The order Verbenacea contains the beautiful flowers cultivated in our gardens under the name of verbena [Latin vervain], and the shrub common not only in greenhouses but in cottage windows, which generally receives the same name, remarkable for its orange-like fragrance. It contains also the teaktree of India, one of the most valuable of timber-trees.-The last order which we shall notice is Primulacea, an order consisting of herbaceous plants, none of which are of large size, but which generally have very beautiful flowers. They are mostly natives of temperate parts of the northern hemisphere. The primrose, the cowslip, the auricula, and the pimpernel are examples of this order.

1 From primula, the primrose, from Latin primulus, very early, so called from its flowering in the beginning of spring.

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GEOLOGY.

Nature of the Subject.

WE have most of us stood at the base of a great cliff, and looked upwards with awe at the rocks exposed on its weathered front. Such a sight might suggest many strange and interesting inquiries. How did these rocks come to be where they are? Of what are they composed? When were they formed? Whence the material for the vast thickness of rock that composes the crust of the earth? Whence have come the varied substances that form our limestones, coals, and sandstones? Whence also the strange shells, plants, and animals that a closer examination of their structure reveals? Are these the remains of bygone living organisms, or are they only marks in the rocks themselves? If they were once living creatures, what were their structure and habits? Such questions suggest themselves to every thinking person, whether man or boy, and such questions Geology undertakes to answer; and it is to the principles of this great science that we now proceed to direct attention.

Geology, from the Greek ge, the earth, and logos, a description, is, according to its name, a description of the earth. It examines the various rocks that compose its crust, and seeks to explain their appearance, form, structure, relative position, formation, age, and distribution throughout the globe. It also inquires minutely into their contents, animal, vegetable, and physical; the causes of their imprisonment in their stony tombs; and the structure and habits of the creatures there found. It pictures forth the physical history of the globe during the successive epochs through which it has passed, with their varied scenery and inhabitants, the formation of its many strata, and the structure and progress of the organic forms that successively waved in its atmosphere, moved over its surface, or swam in its seas. In short, it is the province of Geology to describe the whole natural history of the globe during the various ages of the long past; and it includes the zoology,

botany, mineralogy, and geography of the ancient earth, whose present conditions are the result of the numberless changes through which it has passed in these geological eras. The past it seeks to interpret solely by the present, assured that the laws of nature are invariable and universal, and that causes operating now produced like effects in the primeval earth.

Rocks, their Kinds, Structure, and Disposition.

In order to speak with precision in our study of this subject, it is necessary to have a distinct idea of what a rock is in Geology, and to understand certain things regarding their kinds, structure, and arrangement.

What a Rock is in Geology.-In Geology, the word Rock has a wider meaning than it has in common language, where it means a mass of stone of considerable size. In this science, the word Rock is used to designate any of the materials that compose the crust of the earth, of whatever size and softness they may be. Geologists reckon sandstone, marble, quartz, granite, and limestone to be rocks, as others do; but they also speak of coal, gravel, chalk, sand, salt, peat, and like soft and broken substances, as rocks or rock-formations.

Kinds of Rocks.-Rocks have different names, according to their appearance and structure. Every one knows what sand is, and that it varies greatly in fineness. The most of the sand we see is composed of small particles of rock ground to powder, but it often consists, as we shall afterwards learn, of numberless very minute shells. Sandstone is the usual rock of which houses are built, and which, in thin layers, is used for pavement. This rock is more common than any other, and has many varieties, and is, of course, so called because it is composed of particles of sand that have been made to cohere. When the particles of the sandstone are somewhat larger and sharper, the rock is called grit, from the particles having been grated down or broken the rock of which millstones are formed is called millstone-grit, and its value depends on the hardness and sharpness of the grains of which it is composed. When the particles are larger still, and form small stones that do not cohere, the rock is called gravel; and when yet larger and more rounded, shingle, examples of both of which occur on the sea-beach. A mass of broken angular stones thrown up in a heap, as by a river after a flood, is called rubble. A stone when small is called a pebble; when large, a block; and when rounded and worn, a boulder, because it is ball-shaped. The fine sediment at the bottoms of rivers, lakes, and pools is composed of ground mineral, animal, and vegetable matter. When this is tough and plastic, it is called clay,

because it cleaves or sticks; and the whole accumulation of mud, clay, and sand at the bottom of any water, is called silt.

The remains of vegetable matter found in various parts of the country, and used as fuel, are known as peat; and coal is nothing but such vegetable matter changed by heat, and hardened into rock by pressure. Limestone is the name given to the hard rock which, after being burned in a kiln, forms lime. When the limestone is hard and crystalline, it forms marble, which is of different colours, from deep black to pure white, and often beautifully variegated. Chalk is a variety of limestone, and obtains its name from this fact; the word chalk being another form of the Latin calx, lime.

Common slate, used for writing on and for roofing, is composed of thin layers of hard rock, of which some of our highest mountains are formed. The name shale is applied to a kind of rock which shells off or splits into very thin layers, and which may be seen in great heaps near coal-pits. Thin layers of sandstone used for pavement are called flags. The white pebbles so common on the sea-beach, and so easily broken, are made of quartz, and rock formed of it is called quartz-rock. Some varieties of quartz, called rock-crystals, are very beautiful and valuable, and become even precious stones, such as agate, amethyst, and topaz. Flint has much the same composition as quartz, and is very plentiful in chalk. The granular rock brought from Aberdeen and elsewhere, so beautiful when polished, is called granite, from its being composed of grains of other rocks. The particles that glitter like silver in the granite are pieces of mica,1 which is so named because it shines. A mineral very like mica in appearance, but different in composition, is called talc, from its feeling somewhat greasy or tallowy when touched.

4

The molten matter that flows from volcanoes is called lava;2 pumicestone3 is the cinder of such discharges; while the ashes that are thrown into the air are called scoria. In geologic times also there existed volcanoes from which lava issued; the rock this lava formed is called trap, from lying in stair-like masses, as it flowed from the mountain; and one kind, whinstone, which is much used for roads. A common variety is known as greenstone, from its colour, of which Salisbury Crags, near Edinburgh, are composed. Another variety is called basalt, and is generally found in columns standing close together, which often form wonderful natural scenes, such as Fingal's Cave and the Giant's Causeway. Another variety is porphyry,5 so called from its frequent purple colour, and is easily distinguished by its granular appearance. A kind of light porous rock, formed of cohering volcanic

1 From Latin mico, to shine.

2 From Latin lavo, to flow.
3 From Latin pumex, -icis, cinder.

4 From Latin trappa, a stair.
5 From Latin porphyra, purple.

ashes, is known as trap-tuff, or tufa, a word that comes from Italy, the seat of so much volcanic action.

Structure of Rocks.-On examining the rocks forming the crust of the earth, we find that they may be divided into two great classes—the stratified, or those deposited in strata or layers; and the unstratified, or those not so formed. Sandstone and slate are stratified rocks; granite and trap are unstratified.

1. Stratified Rocks.-Any thin deposit of rock is called a layer, from its having been laid down under water; a band, from its being like a thin band; a bed or a stratum, when of greater thickness, from Latin sterno, to spread; and a seam, when of a peculiar character as compared with the rocks near it, as a seam of coal. Stratum, with the plural strata, is the general term for any layer of rock, and hence all rocks in layers are said to be stratified. Rocks that split up into very thin layers, a great number being included in the thickness of an inch, are said to be laminated. When a rock is composed of rounded pebbles or boulders embedded in other matter, it is called a conglomerate, and sometimes, from its appearance, pudding-stone or plum-pudding-stone.

2. Unstratified Rocks.—Unstratified rocks assume various forms, according as they have been shot up amongst the stratified rocks; for, as we shall afterwards see, they have been erupted from volcanoes. Very often, like most volcanic substances, they are porous or cellular, like pumice-stone; frequently they stand together like gigantic columns, when they are said to be columnar, like basalt; and often they are found in large globular or spherical masses, like bombs or cannon-balls.

Disposition of Rocks.-1. Stratified Rocks.-When rocks lie parallel to the horizon, they are termed flat or horizontal, as A, in the following section; when at an angle to it, they are said to be inclined or dipping, as B; when one end has been thrown up by some other mass, they are said to be tilted up, as R; when so much inclined as to be straight up and down, they are said to be perpendicular, or to stand on edge. When inclined rocks come to the surface, they are said to crop out, and the exposed edge is therefore termed the outcrop, as M; the angle at which they are inclined is called the dip of the rocks, and is measured by the number of degrees from the horizontal in any direction, as 60° S.; and the line of the outcrop along the surface is termed the strike or line of strike, because it strikes or runs across the country. When the strata are not straight, they are said to be bent or curved; and when greatly bent, twisted or contorted, as P. When all the strata in a series lie at the same angle, they are called conformable; when at different angles, unconformable, as at A. Sometimes certain strata seem to have slipped or to have moved up, so that rocks that should be opposite to one another are not The portion that has slipped is naturally termed a slip, as C;

So.

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