nS on MM - 01 in! O-OBOLOGY 



presents no marked inequalities, l>ut where, oevertbe 

 less, a decided widening or narrowing of the inl 

 between two outcrops occurs, we infer with confidence 

 that the intermediate strata must increase or diminish in 

 thickness. 



Estimation of Thickness of Strata.- When the angles 

 of dip have been observed along a line perpendicular to 

 the strike, it is easy to calculate what the thickn. 

 rock must be in any given interval of the section, or to 

 obtain it by using the protractor. This is most 

 veniently done at home, where the observer can collect 

 his notes and protract the angles he has taken in the 

 field. It is useful, however, to have a ready means of 

 estimating thickness, and in this respect the following 

 rule, given many years ago by Maclaren, 1 will be found 

 of service. If the breadth of inclined strata is measured 

 across their outcrop at right angles to the strike, their 

 true thickness will be equal to -jVth of their breadth at 

 the surface for every 5 of dip. Or it may be put thus : 

 divide 60 by the angle of dip, and the fraction is obtained 

 which expresses the thickness. Thus, suppose a mass of 

 strata measures across the strike, 1200 feet, and is 

 uniformly inclined at an angle of 5, its real thickness 

 will be jVth, or 100 feet; at 10 the thickness will be 

 Jth, or 200 feet; at 15 it will be Jth, or 300 feet; at 

 20, Jd, or 400 feet This rule is very nearly accurate 

 for inclinations up to 45. 



Thinning away of Strata Overlap. It sometimes 

 happens that two lines of outcrop come together, owing 

 to the complete thinning away of the intermediate strata, 



Charles Maclaren's Geology of Fife and the Lothians, p. xv. 



