'IIIE nUlI.DINf; OK AN ISLAND. 



19 



As \vr shall find later, this question of dip takes a very important place in 

 the studv of the history of the rock formations, and is, therefore, worth special 

 attention. Occasionally the layers are horizontal and then there can he no 

 question of dip; hut commonly this is not the case, the rocks generally slope 

 in some definite direction, that is tf) say, towards some definite point of the 

 compass, and this is noted down as the dip; the layers mentioned ahove, for 

 example, have a souihwest dip. The amount of the dip is measured in angular 

 degrees from the horizontal, and after a little practice we can generally tell by 

 the eye alone the approximate amount of the dip; hut if we wish to measure 

 it more accuratelv, we must use a simple instrument called a clinometer (slope- 

 measurer). Such an instrument can be bought from an instrument maker;, 

 but may also be constructed at home on a card with sutlicient accuracy for 

 amateur purposes. The figure below shows such a home-made clinometer. 



The Card A B has a semicircle drawn on it, on the edge of which the 

 degrees from one to ninetv are marked off on both sides in an upward direc- 

 tion, starting at D. From the centre C a small shot is suspended by a thread, 

 and the lower edge of the card is applied to the layer of rock. The shot, of 

 course, hangs perpendicularlv antl shows the angle at which the layer under 

 examination slopes from the horizontal. As the beds of rock often present a 

 rather rough surface, it will be found convenient first to rest a straight stick 

 along the rock in the direction of the dip and then to apply the clinometer to 

 the stick, instead of to the rock itself. 



