STRATIFICATION. \.\ 



globe has not sensibly changed, and by the calculations of the 

 learned, it is proved that the surface of the earth receives from the 

 sun during a year a quantity of heat equivalent to that which it 

 loses in the same space of time ; the internal heat of the earth no 

 longer influences the temperature of its surface, except in an in- 

 sensible degree, and to diminish this influence, which is almost 

 none at all, one-half, would require the lapse of 30,000 years. 



8. Our knowledge of the central portion of the globe is limited 

 to what we have just said of its weight and temperature ; but the 

 solid crust, constituting its surface, has been better studied. 



9. This crust is not formed of a single piece, but is composed 

 of a great number of various materials. The study of these vari- 

 ous substances, particularly, belongs to Mineralogy ; the study of 

 their mutual relations and the more or less important part they 

 play in the constitution of the globe, is the province of Geology. 



10. In general we give the name of rocks to mineral substances, 

 which are united in great masses, and apply the term formations, 

 to diverse assemblages of rocks which appear to have oeen formed 

 under the same circumstances. 



The word rock, as used by geologists, is applicable to all mineral masses 

 whether hard or soft, and therefore includes in its meaning, sand, marble, 

 clay, granite, &c. 



11. When we examine the sides of mountains, artificial exca- 

 vations, and various other localities favourable to geological studies, 

 we very soon perceive there are a great many different formations, 

 and these formations are in layers or stories reposing one above 

 the other, constituting strata : (plural of stratum, a Latin word, 

 meaning a bed, couch, or layer ; anything spread out or strewed 

 over a surface.) 



12. We can be convinced of this by examining the cuts made 

 through hills for the passage of rail-roads and canals in various 

 parts of the tlriited States. By comparing the different materials 

 composing the earth's crust, the geologist will soon be satisfied that 

 these different rocks, in a majority of instances, are not placed one 

 alongside the other, but cover each other, and form a series of 

 layers, of more or less thickness, comparable to the courses or 

 layers in a mass or wall of mason-work. Gypsum, or plaster of 

 paris, for example, rests upon a stratum of coarse limestone, for, 

 in digging wells in the neighbourhood of Paris, at different points, 

 ihe coarse limestone is always found below the plaster. This 



8. What do we know relative to the centre of the earth ? 



9. What is the crust of the earth ? Does it consist of one piece ? What 

 is mineralogy ? 



10. What are rocks ? What are formations ? 



11. What is meant by stratum ? 



12. How are rocks placed relatively to each ether 7 



2 



