graduated are with a pendulum is inserted. When the instrument 
is held horizontally, the pendulum points to zero. When placed 
vertically, it marks 90°. By retiring at a right angle to the direc- 
tion of dip of a group of inclined beds, and holding the clinometer 
before the eye until its upper edge coincides with the line of 
bedding, we readily obtain the amount or angle of dip. In observa- 
tions of this nature, it is of course necessary either to place the 
clinometer strictly parallel with the direction of dip, or, if this be 
impossible, to take two measurements, and calculate from them 

























































Fic. 227.—APppaARENTLY HorizonTaL Strata (B.). 
the true angle.” Simple as observation of dip is, it is attended 
with some liabilities to error, against which the observer should be 
on his guard. A single face of rock may not disclose the true dip, 
especially if it be a clean-cut joint face. In Fig. 227, for example, 
the strata might be supposed to be horizontal ; but another side view 






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Fia. 228.—Reau INCLINATION or STRATA SHOWN IN Fic. 227 (B.). 
of them as (Fig. 228) might show them to be inclined or even 
vertical. 
Again, a deceptive surface inclination is not unfrequently to be 
seen among thin-bedded strata. Mere gravitation aided by the 
’ In Jukes’ “Memoir on the South Staffordshire Coal-Field,” in Memoirs of Geo. 
Survey (2nd edit. p. 213), a formula is given for calculating the true dip from the 
apparent dip seen in a cliff. A graphical method of computing the true dip from 
observations of two apparent dips has been suggested by Mr. W. H. Dalton, Geol. 
Mag. x. p. 332. 
. ME we «a 
care | j Pak a ee 
MRT 
510 GEOTECTONIC (STRUCTURAL) GEOLOGY. [Boox IV. 

