

CHAPTER? XEX 
GEOLOGICAL SURVEYING—continued 
Forms of Outcrop. Measurement of Thickness of Strata. Thickening 
and Thinning of Strata. Unconformity. Overlap. Normal Faults. 
Reversed Faults. Eruptive Rocks and Contact Metamorphism. 
Regional Metamorphism. Archzean Gneissose Rocks. 
Forms of Outcrop.—The form and direction of an out- 
crop naturally vary with the configuration of the ground and 
the direction and angle of dip. As a rule, the most winding 
and sinuous outcrops appear among horizontal strata, especially 
when these have been deeply trenched and eroded. Gently 
inclined strata also frequently yield very sinuous outcrops, 
while the outcrops of steeply inclined and vertical beds are 
usually more regular in their trend, and sometimes run for 
long distances in approximately straight lines. 
Florizontal Strata.—In the case of an undulating plateau 
built up of horizontal strata, and traversed in different direc- 
tions by many valleys, the outcrops necessarily follow all the 
windings of the latter—they play the part, in short, of contour- 
lines. The width of the outcrops is determined, of course, 
by their position with regard to the configuration. Thus, 
upon a steep slope, an outcrop of a stratum many feet or 
yards in thickness will be indicated upon the map by a 
relatively narrow band or ribbon, while the outcrop of the 
same stratum occurring on the top of a hill would be repre- 
sented by the whole surface of the bed, which might form 
quite a broad patch of colour on the map. 
Inclined Strata—The outcrops of inclined strata also vary 
in direction with the shape of the ground, but they are 
influenced likewise by the angle of dip—an influence which 
becomes less and less marked as the dip increases, The 
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