To determine the direction of dip a pocket compass is very 

 useful. It is, according to the above definition, at right angles- to 



the water line, as shown 

 in Fig. i. There are 

 various ways of determin- 

 ing the amount of dip. 

 Hold a level (see Fig. i) 

 over the arrow. Measure 

 with a protractor or grad- 

 uated limb of any kind 

 ^ l £- T - the angle made between 



the level and the arrow, this is the amount of dip expressed in 

 degrees. Again (see Fig. i) measure up from water line any 

 convenient number of feet or metres, say 15. There drop a per- 

 pendicular to the surface of the water, say 6 l / 2 feet or metres.' 

 Then 6^-^-15 expresses the rate or amount of dip of these rocks. 



In the ordinary rock exposures one rarely finds the surface 

 of layers or beds laid bare to any considerable extent; it is much 

 more common to see them side-view, as in Plates I, III and IV. 

 Plate III shows how the different layers if traced from "a" 

 to "c" descend toward the right. This descent may or may not 

 be the whole amount of dip of these layers. So far as the ob- 

 server is concerned there may be a sloping or dipping cliff- wards 

 or lake- wards as well as towards the right. Observations in 

 gorges or cuts in other directions will enable the observer to de- 

 termine the true dip of these rocks. Dip being a matter of the 

 inclination of a plane, it is obvious that at least three points not 

 in a straight line must be determined in order to fix the position 

 of the plane. (See observation on opposite page.) 



Considerable stress is here laid on the subject of dip, for as 

 will be apparent upon due consideration of the subject, it fur- 

 nishes the means of estimating the thickness of groups or forma- 

 tions where they have never been drilled through and measured 

 vertically. In southern New York west of about the meridian of 

 Biughamton the rocks dip to the southward at the rate of from 

 25 to 50 feet per mile, averaging perhaps 40 feet. If then one 

 starts at Cortland and walks due south 50 miles and is at the same 

 level above sea as when he started he knows he is on rocks strati- 

 graphically about 40x50 ft. higher than those at Cortland. 



