128 THE BOOK OF NATURE STUDY 



break up into blocks in directions usually at right angles to the 

 bedding and to one another (Figs. 8, 12, 32, and 74). They are very 

 well seen in lumps of coal. They are sometimes the result of 

 shrinkage on drying, and when this is the cause they are related 

 to the hexagonal columns produced by shrinkage in cooling 

 volcanic rocks. But the more regular jointing of sediments is 

 probably due to alternate pressure and tension resulting from the 

 general elevation of consolidated sediments. Joints are of much 

 importance in roughly shaping blocks for building, and for guiding 

 the directions along which excavation is easiest in quarries and 

 mines. Jointing is also very important in giving directions along 

 which water travels through rocks. In a soluble rock like limestone 

 the traversing waters often widen out the joints by solution into 

 open fissures (Fig. 33), and eventually into systems of caves along 

 which much of the drainage is carried underground. Numerous 

 examples may be seen in any limestone district, where the sur- 

 face is often honeycombed with deep hollows, and all the rainfall, 

 unless in exceptionally rainy seasons, disappears underground 

 (Fig. 74). Mechanical denudation also works along the joint 

 planes, and mountain and valley forms are often defined by them. 



Often the rock breaks along joints, and under the pressure or 

 tension one side of the fissure travels up or down relatively to 

 the other. Such movement planes are known as faults. Folds 

 often pass into faults, and the two phenomena are closely connected 

 by causation with one another, and both are related to the causes 

 which have produced jointing. Faults, too, often produce striking 

 results in the landscape of the country in which they occur. 



At their contact with igneous rocks the clastic rocks are often 

 found to be much altered in chemical and mineralogical characters ; 

 they are usually hardened and rendered more compact and less 

 pervious to water. This is spoken of as metamorphism, and it has 

 often proceeded so far that the metamorphic rocks have become 

 crystalline. This is well seen at the margin of granites, or other 

 large masses of crystalline rocks. Similar effects have often been 

 produced in large masses of rocks, such as those of the Scottish 

 Highlands, when these have been brought within the influence of 

 the extreme heat of the earth's interior. In this case the rocks have 

 become crystalline throughout, and are known as schists and 



