away with them immense rocks and masses of earth, which they deposit in distant parts of the ocean.

11. Volcanoes and earthquakes are the most terrible in their effects of all geological agencies; but the actual geological changes which they produce are much less important than those occasioned by what are apparently the most insignificant of animals—the little coral polypes, shell-fish, and invisible animalcules. The latter minute organisms, so small that millions of them might sport freely in a drop of water, are now, as in ages past, important geological agencies, floating in the air we breathe, adding to the soil we cultivate, and forming vast layers of rocky strata at the bottom of the

ocean.

12. The rotten-stone or polishing powder, called tripoli, is composed of the flinty shells of animalcules; and there are extensive marl-beds in our country composed of similar materials. Eminent geologists have expressed the belief that all the lime of our marble-quarries and chalk-beds has been formed of the shells of organized bodies-probably deposited at the bottom of the ocean, as lime-beds are now forming there.

"The earth that's Nature's mother is her tomb,"

is the scientific assertion of Shakspeare; and even the contemplative Young inquires,

"Where is the dust that has not been alive?"

13. The coral-building animals have been partially described in the division on Physical Geography; but we are now prepared to regard them as the most important of all geological agents. They are actually filling up portions of the Pacific Ocean: coral islands are now numbered there by thousands, and coral reefs are hundreds of miles in extent; and if the slow operations of these little animals shall continue as long as some of the geological periods which we have noticed, the result will be a new continent there.

LESSON XIII.-CORAL ISLANDS.

[See Illustration, p. 371.]

1. I SAW the living pile ascend,

The mausoleum of its architects,

Still dying upward as their labors closed;
Slime the material, but the slime was turned
To adamant by their petrific touch.

Frail were their frames, ephemeral their lives,

X

Their masonry imperishable.

All

Life's needful functions, food, exertion, rest,
By wise economy of Providence,

Were overruled, to carry on the process
Which out of water brought forth solid rock.
2. Atom by atom, thus the mountain grew
A coral island, stretching east and west;
Steep were the flanks, with precipices sharp,
Descending to their base in ocean gloom.
Chasms few, and narrow, and irregular,
Formed harbors, safe at once and perilous-
Safe for defense, but perilous to enter.
A sea-lake shone amid the fossil isle,
Reflecting in a ring its cliffs and caverns,
With heaven itself seen in a lake below.
3. Compared with this amazing edifice,
Raised by the weakest creatures in existence,
What are the works of intellectual man,
His temples, palaces, and sepulchres?
Dust in the balance, atoms in the gale,
Compared with these achievements in the deep,
Were all the monuments of olden time.

4. Egypt's gray piles of hieroglyphic grandeur,
That have survived the language which they speak,
Preserving its dead emblems to the eye,

Yet hiding from the eye what these reveal;
Her pyramids would be mere pinnacles,

Her giant statues, wrought from rocks of granite,
But puny ornaments for such a pile

As this stupendous mount of catacombs,
Filled with dry mummies of the builder-worms.

MONTGOMERY.

LESSON XIV.-GEOLOGICAL MONUMENTS.

If we look with wonder upon the great remains of human works, such as the columns of Palmyra, broken in the midst of the desert, the temples of Pæstum, beautiful in the decay of twenty centuries, or the mutilated fragments of Greek sculpture in the Acropolis of Athens, or in our own museums, as proofs of the genius of artists, and power and riches of nations now passed away, with how much deeper feeling of admiration must we consider those grand monuments of nature which mark the revolutions of the globe-continents broken into islands; one land produced, another destroyed; the bottom of the ocean become a fertile soil; whole races of animals extinct, and the bones and exuviæ1 of one class covered with the remains of another; and upon the graves

of past generations-the marble or rocky tombs, as it were, of a former animated world-new generations rising, and order and harmony established, and a system of life and beauty produced out of chaos and death, proving the infinite power, wisdom, and goodness of the GREAT CAUSE of all things.— SIR H. DAVY.

: Ex-U'-VI-Æ (egz-yu'-ve-e), whatever is put off, or shed and left, by animals or by plants; the cast skin, shells, etc., of animals.

LESSON XV.-MINERALOGY: THE ALPHABET OF GEOLOGY.

SIMPLE MINERALS.

1. In the language of geology, all natural bodies that are neither animal nor vegetable are called minerals. In this view, not only are all kinds of clay, stones, and the metals to be considered minerals, but water also must be included in the list. If the earth were sufficiently heated, the rocks themselves would melt and flow like water, as we see in the case of melted lava; and, if the earth were sufficiently cold, we should rarely see water, except in the rock-form of crystallike masses of ice.

2. Mineralogy, therefore, whose subject is minerals, treats of all the inorganic substances that are found in the earth or on its surface; it arranges and classifies them, it designates the ingredients of which they are composed, and it describes their properties. Hence minerals are the very alphabet of geology; and mineralogy is only a branch of that science whose grand historical outlines we have just been considering.

3. Of what, then, are the materials of the earth composed? It would seem, at first view, that they must be almost infinite in number and variety; that a thousand kinds of stone and earth might easily be gathered, and that no limits could be assigned to the extent of such a geological collection. But a little examination shows that this vast multitude of seemingly different kinds of rock and earth, and clayey and marly soils, is composed of only a few primary ingredients, although they are combined in a great variety of forms and propor

tions.

4. Mineralogy, no less than geology, is full of wonders; one of the greatest of which is that the life-sustaining oxygen which we breathe so freely in pure air enters so largely into the composition of rocks and earths as to constitute nearly one half of their solid materials. This is the first, sim

ple, but all-important lesson in mineralogy. A second lesson teaches us that, out of nearly sixty pure mineral substances which are known, six of them, although seldom obtained in a separate state, are found so largely combined with this same oxygen as to form, in this compound state, nineteen twentieths of all the rocks and earths which are known. Thus ten twentieths of the inorganic parts of our globe are composed of oxygen; six mineral substances go to make up nine twentieths more; and the remaining one twentieth is composed of other minerals.

5. The six mineral substances, or mineral bases, to which we have alluded, have been named silicon, calcium, aluminum, magnesium, potassium, and sodium. Thus silicon unites with oxygen in certain proportions to form the well-known and abundant flinty or quartz rock. When quartz is broken down into fine grains, and consolidated or cemented with oxyd of iron, it forms sandstone rock; and, in the form of finely-powdered sand, it is an important ingredient in the soil we cultivate. Calcium and oxygen form lime; and when this is united with carbonic acid, the result is limestone rock, which is also an ingredient of our best soils. Similar combinations of the other mineral bases with oxygen form alumina, magnesia, and soda, which also enter to a considerable extent into the composition of the rocky and earthy portions of the globe.

6. The most abundant of the simple minerals, or rocks, as they are generally called, which are formed chiefly by the simple union of oxygen with the six mineral substances mentioned, but in some instances by additional combinations, are quartz, feldspar, limestone, hornblende, mica, talc, and serpentine; and these are distinguished and described by their color, and their several degrees of lustre, transparency, specific gravity, hardness, fracture, tenacity, taste when soluble, and odor when rubbed.

7. Of these minerals, quartz, which enters largely into the compound minerals or rocks, constitutes nearly one half of the rocky crust of the earth. Pure quartz, which is silica, scratches glass with facility, although it is inferior to the diamond in hardness. Flint and rock-crystal are well-known forms of this mineral, but it occurs in numerous other varieties; and when colored by iron, manganese, chrome, and other foreign substances, it produces many valuable gems or precious stones, such as opal, jasper, amethyst, agates, and carnelians. The sand which is used in making mortar and glass

is mostly quartz; and in what is called silex this mineral forms the hard, flinty covering of the grasses.

8. Feldspar, or fieldspar, whien contains a large proportion of alumina, the basis of clay, composes about one tenth of the crust of the globe. It is of various colors, is not so hard as quartz, and is less glassy in appearance. It is used extensively in the manufacture of porcelain. Common clay is impure de. composed feldspar.

9. Limestone, forming about one seventh of the crust of the earth, presents numerous varieties-from the common chalk, cavern stalactites, and coarse limestone rock, to the beautiful crystalline spars and the finest marble.

10. Hornblende, a tough mineral, as implied in the name it bears, constitutes a large part of the rocks of volcanic origin and some of the older slate rocks. It forms about one fif teenth of the crust of the earth. One of its varieties is the remarkable asbestus, whose slender fibres may be woven into cloth which will be incombustible.

11. Mica, often improperly called isinglass, is a soft mineral, usually of a light green color, and is about as abundant as hornblende. Thin plates of it are often used for lanterns and stove windows.

12. Tale resembles mica, but is softer, and may be easily cut with a knife. Steatite, or soap-stone, one of its varieties, is extensively used for fireplaces and stove linings.

13. Serpentine, which is of various colors, is harder than limestone. Its finer varieties, which admit a high polish, are an elegant substitute for marble.

14. Gypsum, or "plaster of Paris," rock-salt, and coal, complete the list of minerals which form any considerable portion of the earth's crust.

LESSON XVI.-COMPOUND MINERALS.

1. GRANITE, which forms so large a proportion of the primary rocks, is a crystalline aggregate of quartz, feldspar, and mica. It is a very hard and durable rock, and is much used in building and for pavements. Granite seems to be the general foundation-stone or underpinning of the other rocks, and it is also found as high as the summit of Mount Blanc.

2. Granite is abundant in New England. The most celebrated quarries are at Quincy, Mass. The Quincy granite, however, is not properly a granite, but a syenite, in which