THE STRUCTURE OF THE EARTH 



253 



^■'■wW'.v,* 



plane, it has a 30 dip ; if the angle is 40 , the dip is 40 . The ridge- 

 pole of the house extends at right angles to the dip of the roof 

 and corresponds to the strike of a stratum which is defined as 

 the direction at right angles to the dip. If, for example, a bed dips 

 to the west, the strike is north and south (Fig. 239). The importance 

 of ascertaining the dip and strike 

 of beds is evident in such cases as 

 the following. Suppose a land- 

 owner finds that a valuable coal 

 seam outcrops a few hundred feet 



east of his property. If the dip of and strike, the dip being the angle of the 

 the bed were found to be 15° west, greatest inclination of a bed (shown by 

 1 , t , 11 ! arrow), and the strike the direction at 



he would know that the coal seam rIght angles to the dIp 



could also be encountered on his 



property by sinking a shaft, and the exact depth at which it would be 



found could be determined by a simple mathematical calculation 



(Fig. 240). As the angle BAC (dip) is 15 and the angle ABC, a 



right angle, the angle BCA must be 75 . The length of the side BA 



BC 



or 



Fig. 239. — Diagram illustrating dip 



can be ascertained by the trigonometric formula, tan BAC 



AB' 



BC = tan BAC X AB. If, however, it were found that the strike 

 of the coal seam is east and west, the landowner would know that 



in this case also the 



coal seam probably 

 extended on his land 

 and was merely hid- 

 den from view by soil 

 or glacial drift. A 

 knowledge of the dip 

 and strike of a water- 

 bearing stratum, 

 such as the Dakota 

 has enabled those in 



Fig. 240. — Block diagram showing the effect of dip 

 upon the width of the outcrop of strata. The strata at 

 the right are nearly four times as wide as those at the 

 left, although the thickness is the same. 



sandstone in Nebraska and South Dakota 

 search of artesian water to estimate the depth to which it is neces- 

 sary to bore and whether or not a well would flow. It is, however, 

 in determining the structure of a region that the dip and strike are 

 especially used. 



Effect of Dip and Strike upon Outcrop. — When a series of strata 

 are horizontal, only the uppermost appears at the surface ; but when 

 the beds are inclined and eroded, each bed in succession outcrops 



