STKATA. ' 49 



Upon examining various portions of the earth's surface, 



they will be found to differ in character from each other ; in 



one place perhaps sandy soil will be found, in another, hard 



rock, in a third, clay, in a fourth, chalk, and so on. Now, if 



this examination were carried no further, it might well be 



presumed that each of these kinds of soil were continued 



downwards into the earth for an indefinite distance, but 



upon digging (as in sinking a shaft or well) it is found that 



the stratum of soil upon the surface is soon passed through, 



that others in succession present themselves and that this 



succession of strata is not a matter of chance, but (with 



certain restrictions) follows in invariable order. It is true 



some strata are occasionally not present, and that others 



replace them ; but yet (taken altogether) there is the same 



order of succession everywhere found. The following 



diagram (fig. 1) gives the section of these strata from the 



earth's surface to the granite which is the lowest of all 



formations and through which no one has ever penetrated. 



The figures give an approximation to their average thickness 



in feet. These strata do not all run in a direction parallel 



to the surface of the earth, or else it would result that 



the surface would everywhere be made up of the highest, 



but they have been contorted and heaved up into 



mounds or depressed; and the granite has often been 



pushed upwards, carrying with it all the upper strata 



to a certain extent, at last thrusting quite through them 



all and presenting itself above the surface, forming in this 



way the tops of many very high mountains, Mont Blanc 



for instance. Such a mountain if cleft from the summit to 



the base would present a section similar to that shown in 



fig. 2, and thus it occurs (contrary to what would at first 



thought be supposed) that the highest ground is generally 



formed of the lowest strata, while the valleys are nearly 



always covered with the latest formations, which is partly 



owing to the elevating process before alluded to, and partly 



to the winds, rains, &c., tending to carry away and wash 



down every kind of soil from the upper ground and deposit 



it in the lower. 



Fig. 3 represents strata in a state called by geologists 



E 



