Imatges de pÓgina
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produced the convulsions and changes which have taken place both on the surface and in the interior strata of the globe. They are likewise applicable to various practical purposes. In excavations for the purposes of forming canals, tunnels and railroads, operations which are now going forward in almost every part of the civilized world, a knowledge of this subject would not fail to be highly beneficial to all parties engaged in such projects. For these reasons it might be highly expedient to communicate to the young a general idea of some of the leading facts connected with Geology and Physical Geography. The text-book used is Rhind's Elements of Geology and Physical Geography.

EXERCISES.

What does Geology treat of? It gives a history of the various strata composing the superficial crust of the globe. How far has man penetrated into the earth ? It does not much exceed two thousand seven hundred feet. What is the supposed density of the solid mass of the globe ? The mean density has been estimated as five times greater than that of water, and it gradually increases from the surface towards the centre. Is the external form of the earth a sphere? It is a spheroid, being flattened at the poles and elevated at the equator about twenty-six miles. What is the general form of the earth's surface? The surface of the earth is divided into mountains and valleys, and these are intersected by lakes and rivers. What proportion does the land bear to the water? The ocean covers nearly three fourths of the whole. What is the height of the atmosphere surrounding the earth ? It extends to the height of from forty-five to fifty miles, and is the medium by which vapour and heat are diffused. What are the changes

at present taking place on the earth's surface? The action of the atmosphere, of rain, rivers, and the ocean wash away the solid rocks and looser soil from the higher grounds into the depths of the ocean. What is this process termed ? It is termed disintegration, and the fragmentary portions of rocks are called debris. What are the agencies which counteract this gradual decay? Vegetation is one means by which the wearing down of the surface is prevented. Explain the nature of the protection afforded by vegetation. When the verdure decays and renews every year, it adds to the soil, and forms a secure protection to the rock below. Explain the nature of the compensatory powers of animals. A species of zoophytes fix themselves upon projecting rocks in the ocean, and secreting lime from its waters build up the arborescent corals. Describe the formation of coral islands. The coral reefs gradually entangle the floating sea weed trees and plants, and thus islands are formed. At low water, these reefs entangle floating vegetation, till in process of time a soil for the growth of shrubs and palms has accumulated. Are the changes on the old world within the records of history very great or conspicuous ? The actual changes in the old world have been very partial and insignificant. The monuments of Thebes, the famous tower of Pharos, and the sea bath of Eunostæ cut in a solid rock are yet remaining. Scylla and Charybdis and the Isthmus of Corinth are the same as they were three thousand years ago.

What is the nature and formation of peat? Peat is formed of various vegetables, as lichens, moss, reeds, grasses, heaths, and shrubs. Give examples of this formation. It is formed from a large accumulation of spongy vegetable mould, and large trunks of trees are found amid peat. What does diluvium and alluvium mean? Diluvium means an accumulation of clay or sand mixed with rounded fragments of rocks; alluvium means soil of more recent formation. What is the nature of volcanos ? They are eruptions of smoke, heat, and streams of lava from the tops of mountains. What is the usual situation of volcanic mountains ? They are all situated near the sea shore. What is the computed number of existing volcanos, and in what regions of the globe are they found ? In Europe there are only four great volcanos, Etna in Sicily, Vesuvius in Italy, Hecla in Iceland, and Esk Mount in Jan Mayen’s island. What is the nature of a volcanic eruption ? First of all are heard subterranean noises and internal bellowings, with an undulatory motion of the earth, and often large cracks and fissures occur and a sinking of the earth : then the volcanic mountain bursts forth with volumes of smoke and flame from its crater followed by streams of liquid lava. What are the varieties of lava ?

It is composed of silica, alumina, potash, lime, magnesia, and oxide of iron in various proportions. What is the probable cause of volcanic action? There are various conjectures. Sir H. Davy conjectures that at a certain depth below the surface may exist matter of the earth ; that when the water of the ocean or internal springs comes in contact with this matter, such is the violent and extensive production of heat that a volcanic eruption immediately ensues. Describe Etna and its eruptions. Etna is about ten thousand feet in height. The crater is about a mile and a half in circumference and forms an oval. Describe the formation of Graham island. It is near the Lipari islands, on the coast of Sicily; in 1831 it was raised by volcanic agency in the ocean to the height of eighty feet and about five hundred paces in length, but in a few months this island disappeared. Describe Vesuvius. It is on the coast of Italy, of great antiquity. There was a tremendous eruption of this

mountain in the seventy-ninth year of the Christian æra, in which the cities of Herculaneum and Pompeii were completely destroyed, and Pliny lost his life in watching the phænomena. Describe Hecla. This mountain is about five thousand feet in height. The base is of rugged masses of lava and scoria, seventy feet in height, and the summit is covered with eternal snow and glaciers. The crater continually emits smoke and lava. What are Gysers ? They are boiling springs of Iceland. What is the nature of an earthquake? It consists of tremblings and oscillations of the earth, accompanied or preceded by internal noises and frequently by cracks and fissures of the surface. What is the effect of earthquakes on the surface of the globe ? Not unfrequently the surface is elevated instead of being depressed. Are earthquakes mentioned in the early history of the globe ? Yes; from the earliest records of antiquity the world appears to have been convulsed by earthquakes. When did the memorable earthquake at Lisbon take place ? In the year 1755. When did the earthquake at Calabria take place ? In the year 1783. Is South America subject to earthquakes? Yes; earthquakes are of frequent occurrence in South America. Have any earthquakes occurred in England ? Yes; in 1580 St. Paul's in London was injured by a concussion of the earth. In 1777 a slight shock was felt at Manchester, and in 1795 another shock was felt through all England. When have they happened in Scotland ? In 1816 an earthquake occurred in the north of Scotland, and slight shocks have been felt at Comrie in Perthshire. What is meant by temperature? The degree of heat at the earth's surface. What is the chief source of heat ? The source of heat thus diffused over the globe is derived from the sun. Does the temperature vary, and what is the medium degree of heat? The medium temperature over the globe has been estimated at 56o. Does the temperature vary at different altitudes? The tempera. ture decreases gradually as we ascend higher in the atmosphere. At a certain height we come to a region of perpetual congelation, where water always freezes and produces ice and snow. What are the different opinions regarding central heat ? Some say there exists a central heat within the globe, but we know that the sun's rays greatly influence the temperature at the surface, so that the equatorial heat should be much greater than it is if there were a central heat. What is Mr. Lyell's theory of the change of temperature? Mr. Lyell proposes to account for the change of temperature by the relative change of the land and sea over considerable portions of the earth's surface. What proportion of the surface of the earth is dry land? One fourth of the surface of the globe consists of dry land. What is remarkable in the position of this dry land ? The amount of land is about thirteen times greater on the northern side of the equator than on the southern. Is the dry land always above the ocean level ? The whole earth has a greater or less elevation above the level of the sea. What has been the cause of the elevation of mountains ? Mountains owe their origin to eruptions of matter from below, and they vary their structure according to the particular strata from whence they have been formed. How does the elevation of mountain ranges affect rivers ? From each side of the great chain of the Alps and Carpathian mountains certain rivers take their rise, which flowing in opposite directions, discharge their waters into different seas. Which are the most elevated chains in the world ? The Himmaleh mountains, in the north of India, which are 27,677 feet high; the Andes of South America are 25,250 feet. What is the highest land in Europe ? Mont Blanc 15,665 feet high. Which is the highest

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