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514 GEOTECTONIC (STRUCTURAL) GEOLOGY. [Boox IV 
dip it is possible to estimate the thickness of a series of beds, and how far 
beneath the surface any given bed might be expected to be found. If, 
for instance, the horizontal distance across the strike between beds s and 
a (Fig. 231) were found to be 200 feet, with a mean dip of 15°, the actual 
thickness would be 51°8 feet, and bed a would be found at a depth of 53:8 
feet below the outcrop of s. If the same development of strata continues 
inland, the bed a should be found at a little more than 200 feet beneath the 
surface if a bore were sunk to it in the quarry (Q). If the total depth ~ 
of rock between a and b be 1000 feet, then evidently, if the strata could 
be restored to their original approximately horizontal position, with bed 
a at the surface, bed b would be covered to a depth of 1000 feet. It 
will be noticed also that as the angle of dip increases, the outcrops 
are thereby brought closer together. Where the outcrops run along the 
face of a cliff or steep bank (B) they must likewise be drawn together on 
amap. Inreality, of course, these variations may take place though the 
same vertical thickness of rock everywhere intervenes between the 
several outcrops. 

Fic. 232.—SEcTIon ALONG THE Line L L’ Iw Fria. 231. 
It is usually desirable to estimate the thicknesses of strata, especially 
where, as in Fig. 231, they are exposed in continuous section. A convenient 
though not strictly accurate rule for this purpose may be applied in cases — 
where the angle of inclination is less than 45°. The real thickness of a 
mass of inclined strata may be taken to be +, of its apparent thickness for 
every 5° of dip. Thus if aset of beds dips steadily in one direction at 
5° for a horizontal space of 1200 feet measured perpendicularly to the 
strike, their actual thickness will be ~;, or 100 feet. If the dip be 15°, 
the true thickness will be 3,, or 300 feet, and so on.! 
Part LV.—CuRVATURE. 
_ A little reflection will show that though, so far as regards the 
trifling portions of the rocks visible at the surface, we might re- 
gard the inclined surfaces of strata as parts of straight lines, they 
must nevertheless be parts of large curves. Take for example the 
section in Fig, 233. At the left hand the strata descend beneath 

. QC KG QR RR 
. MK MQ AY NG QU 
AC \ SS 
ia. 233.—Srorion or Inciinep Srrava. 
the surface at an angle of no more than 15°, but at the opposite end 
the angle has risen to 60°. There being no dislocation or abrupt 
* Maclaren’s “Geology of Fife and the “Lothians,” 2nd Edit, p. xix. For tables 
for estimating dip and thickness see Jukes’ “ Manual,” p. 748. 
/ 
