POPULAR GEOLOGY, 



Geology is a branch of science which investigates both the ancient natural 

 history and physical condition of the earth's crust ; treats of the successive modi- 

 fications it has undergone; and the agencies which even now are producing 

 changes on the surface of the globe. Palaeontology, which specially treats of the 

 history and affinities of those animals and plants whose remains occur in the 

 various strata; and Mineralogy, which treats of the composition and actual 

 nature of the materials composing the various rocks and strata, are intimately 

 connected with geology. 



The crust of the earth, up to the altitude of 24,000 feet, and down to depths of 

 3,000 feet, has in every direction of its accessible parts been investigated, and 

 sufficient is known of its structure to warrant the assumption, with tolerable 

 certainty, of the following important principle: The crust of tJie earth consists of 

 only a proportionably small number of different rocks, and these are similar to each 

 other at the most distant parts of the globe, as to their principal mineral characters. 

 Thus the various kinds of rock are distributed over the entire globe, the granites 

 of South America and of the most northern climates are nearly alike; while on 

 the other hand, plants and animals of the equator, of the temperate zones, and 

 of the polar circles, exhibit the most striking differences. 



Heat of the Globe. The temperature of the globe is an important element 

 in the history of the changes which the earth has undergone. At each point of 

 the earth's surface there is a certain mean temperature ; but beneath the surface, 

 observations show that a continual augmentation of temperature proportioned 

 to the depth constantly occurs. It is hence concluded, that the interior parts of 

 the globe are incomparably hotter than the parts at the surface; must formerly 

 have been still hotter, and have influenced to some extent the temperature 

 and all the other phenomena at the surface of the earth. That the internal heat 

 was once greater than it now is, is evident from many facts. The deepest rocks 

 are such as appear evidently to have been cooled down from igneous fusion; and 

 the figure of the earth is such as would result from revolution on its axis, provi- 

 ded the whole or a large part of its mass were in a state of fluidity or viscidity. 



Modern causes Of Change. Besides the changes resulting from the gradual 

 cooling of the mass of the earth, there are many other forces now in action 

 tending to produce changes in the external crust of the globe. The varying heat 

 received from the sun; the effect of heat and physical condition in modifying the 

 animal and vegetable world; the disintegrating effect of seas, rivers, springs, and 

 rain; the chemical and mechanical action of the atmosphere; the disruptive 

 forces of volcanoes and earthquakes; the sediments transported by rivers j the 

 formations due wholly to the labours of innumerable marine animals ; the effects 

 of frost, glaciers, and icebergs all tend to produce incessant change on the 

 earth's surface. These changes affect the geographical boundaries of land and 

 water, the relative levels of land and sea, and the forms, proportions, and distri- 

 bution of organic life. 



The statement of the effects of modern causes of change.oa the earth's surface 

 is also applicable to former eras of the world, at least in its general features; but 

 they may not always have been equal in degree of action. Many sudden changes 

 have evidently occurred, arising from the unusual predominance of some of the 

 above forces. 



Successive Periods Of Formation. At a certain depth below the surface 

 of the earth the rocks are massive, without stratification, and without fossils, 

 affording evidence of having been acted on by heat; but above these rocks are 

 others which, by being stratified, and by having fossils peculiar to themselves, 

 may be classified and arranged. They represent too, epochs of time, in respect 

 to their period of formation, although we are u-nable to measure that time by 

 years or centuries. 



The rocks composing the earth's crust may be classified in various ^vays* 

 Looking merely at the formation of the rocks, we may distinguish Stratified and 

 IJnstratified rocks. If we consider whether remains of plants or animals have 

 been found in the deposits, we may distinguish Fossiliferous and Unfossili- 



