OF CENTRAL CANADA PART III. 



163 



Fig. 88. 



a graduated arc, as in the 

 annexed figure. When the 

 upper edge of the compass is 

 held horizontally, the index 

 cuts the zero point of thegradu- 

 ated arc. From each side of 

 this point, the graduation is 

 carried up to to 90. If, con- 

 sequently, the upper edge of 

 the instrument be placed par- 

 allel with the inclined beds of 

 any strata, the angle of the 

 dip will be at once shewn by 

 the index. A contrivance of 

 this kind, exclusive of the compass, may, be easily made out of a 

 semicircle of hard wood. The index may consist of a piece of twine 

 extending below the graduated limb, and kept taut by a lead plumb 

 or by a stone. 



In a compass used for taking bearings, it is convenient to mark 

 the west side EAST, and the east side WEST, as in the figure. If the 

 north side of the instrument be then kept always in advance, and the 

 angle be always taken from the north end of the needle no matter 

 what the actual direction of the line the true magnetic bearing is 

 obtained at once, and without risk of error. The compass is most 

 readily held by passing the thumb through a short strap or loop, or 

 through a hinged ring, attached to its under side. Where very ac- 

 curate bearings are required, sights may be used, the instrument 

 being fixed on a support ; or a prismatic compass may be more con- 

 veniently employed. 



When strata dip in two directions, as at A, in Fig. 89, the line 

 along the culminating point of the strata is termed an Anticlinal or 

 Anticlinal Axis ; and the line from which the strata rise in opposite 

 directions, as at S in the figure, is called a Synclinal or Synclinal Axis. 

 Synclinals when of a certain magnitude, constitute ' valleys of un- 

 dulation." Anticlinals are often hollowed out by denudation, form- 

 ing valleys or troughs called " valleys of elevation," as shewn at E in 

 Fig. 89. The term " elevation " applies here, it should be observed, 

 to the raised strata, and not to the actual position of the valley, as 



