Explanation of compound terms, like arenaceous shale, etc. — 

 A layer or bed may show characters in common with several of 

 the above-named kinds of rock. For example a shaly layer may 

 contain some grit (sand, arena); it would then be termed arena- 

 ceous shale. It may be also more or less calcareous or limy and 

 would then be spoken of as an arenaceous, calcareous shale. A 

 sandstone may not be very gritty, and it may have considerable 

 shaly matter in its composition and would then be styled an 

 argillaceous or shaly sandstone. A sandstone or shale may be 

 more or less calcareous; a sandstone may become a conglomerate 

 if its pebbles are large. A conglomerate may be sandy or cal- 

 careous, and so on. 



The noun in each case indicates the most important or essen- 

 tial characters of the rock, while the adjectives refer to the less 

 important or casual features. 



Definition and Explanation of a Few Important Geo- 

 logical Terms. 



Joints. — As well shown in Plate III, there are often more or 

 less vertical crevices or cracks traversing the rock layers in one 

 or more directions. These are technically termed joints, and 

 rocks possessing them are said to show a jointed structure. Two 

 sets of joints are shown on Plate III, one nearly parallel to the 

 lake shore, the other roughly at right angles to the first. 



Dip. — That rocks when undisturbed lie in layers of various 

 thickness one above the other, like boards in a pile, has already 

 been mentioned. These various layers are, however, almost 

 never perfectly horizontal, they incline or slope more or less in 

 one direction or another. This inclination or slope is called the 

 dip of the rocks and its amount and direction are determined as 

 follows (see Fig. i): The angle made by any of these layers and 

 surface of the water, or any horizontal surface, is the amount of 

 dip. The direction of any plane that cuts vertically any hori- 

 zontal line drawn on the surface of any layer is the direction of 

 the dip. The direction of the horizontal line drawn on the sur- 

 face of an)* layer is termed the strike of the layer. 



