

CONDITIONS OF STRATIFICATION. 



every student attentive to the subject cannot fail to discover, even in 

 a very limited district, that the different strata which appear above one 

 another are arranged in a certain constant order of succession. A stra- 

 tum which in any one situation is found beneath another will never, 

 in any other situation, be found above it ; in other words, their order is 

 never inverted unless the whole series has been turned upside down. 



Superposition of Strata. As sometimes we neglect to bind in some 

 book a particular leaf, so Nature sometimes omits a particular rock ; 

 but she never misplaces them. Most stratified rocks, when exposed 

 in a sea cliff, quarry face, or river bank, are seen to be composed of a 

 number of parallel planes, or layers, or flat tabular masses which more 

 or less readily separate from each other. These strata or beds, whether 

 composed of sandstone, limestone, or clays, are superposed one upon 

 the other, and are classed under the group of rocks which are bedded 

 or superposed one upon the other. These beds are never misplaced, 

 whatever omission or non-deposition may have taken place; and 

 observation has determined that strata are arranged with respect 

 to one another in a certain constant order of succession. Strata vary 

 in thickness from fractions of an inch to many feet in thickness, 

 whereas laminae seldom occur an inch in thickness, varying from this 

 to the thinness of paper. 



Inclination of Strata. Pursuing our investigation, we find that 

 the strata are generally so disposed that their planes of bedding or 

 broad surfaces of junction with each other are not exactly level or 

 parallel to the earth's spherical surface, but slope in some one direc- 

 tion, so as, in that direction, to sink deeper and still deeper into the 

 earth, and to be covered by other strata. 

 This slope, or deviation from the horizontal 

 position, is called the dip or inclination of 

 the strata; and the rocks are accordingly 

 said to dip or incline to this or that part of 

 the horizon or point of the compass. Dip 

 is the key to the structure of a country, 

 because it acquaints us with the relative 

 antiquity and lie of the beds. It is esti- 

 mated in degrees, but two observations are 

 generally required to find the direction and amount of the dip. Thus 

 in one side of the figure the beds appear highly inclined, and in the 

 other side but slightly inclined. 

 On one side the dip appears to 

 be to the north, on the other 

 side to the east. From these 

 observations the true dip is seen 

 to be NNE. Dip may be at 

 any angle, but if the angle is 

 more than 90 the beds are 

 overturned, and the dip is said 

 to be reversed. Thus owing to 

 folding, the order of the beds on the two sides of the section is re 



Fig. 16. 



