162 



MINERALS AND GEOLOGY 



matter carried off in some districts by denudation, must have beei 

 of enormous amount ; and when it is considered that most of the ine- 

 qualities on the earth's surface those at least not immediately con- 

 nected with mountain chains have been thus produced, the part 

 played by the denuding agencies of former periods in providing the 

 materials of newer strata, may be readily appreciated. 



(c) Tilting up and Fracturing of Strata. Whilst some strata re- 

 tain their original horizontality, others are more or less inclined, and 

 some few occupy a vertical and even a recurved position. That 

 strata were not originally inclined, at least to any extent, is proved 

 by the known arrangement of sediments when diffused through 

 water, these (with the exceptional cases already pointed out) 

 always depositing themselves in horizontal, or nearly horizontal, 

 layers. The same fact is shown also by the frequent presence of 



FIG. 86. 



rows of pebbles, fossil shells, &c., parallel with the planes of stratifi- 

 cation, as in Fig. 86 ; by the occasional presence of the fossilized 

 stems of trees (evidently in their positions of growth) standing at 

 right angles to these planes (Fig. 87) ; and sometimes by the presence 

 of stalactites suspended in a similar position. It is evident that 

 these bodies could not have been originally inclined in this manner 

 to the horizon. 



The inclination of strata is technically termed the dip ; and the 

 direction of the up-turned edges, the strike. The dip and strike are 

 always at right angles. In observing the dip, we have to notice 

 both its angle or amount, and its direction or bearing as north, 

 north-east, N 10 E, and so forth. The direction of the dip is of 

 course ascertained by the compass ; the rate of inclination, by the 

 eye, or by an instrument called a clinometer. The most convenient 

 instrument for both purposes, is a pocket compass, set in a square 

 bed, or attached to a square plate of metal, and furnished, in addition 

 to the needle and graduated limb, with a moveable index. The 

 latter hangs freely from the centre of the compass, and plays around 



