THE CKUST OF THE EARTH. 



which show that, by the violent agitation of the waters consequent 

 on sudden changes of level of their bed, and of the parts of con- 

 tinents over which they have swept, strata already formed have 

 been partially broken up and carried away by currents. Excava- 

 tions, such as that represented in fig. 32, are explained in this 

 manner. 



Fig. 32. 



In such cases it has often happened that the waters under 

 which such broken strata were submerged, having again become 

 tranquil, a new series of strata have been deposited horizontally, 

 filling up the excavation thus formed, as shown in fig. 33. 



Fig. 33. 



89. Closely allied with the matter ejected from volcanoes are the 

 Basaltic deposits, which form so grand a feature in the scenery of 

 many countries with which travellers are familiar. 



All the circumstances and characters which attend these rocks 

 conspire to show that they have issued from openings in the crust 

 of the earth in a state of fusion much more complete than that of 

 volcanic lava, and in the process of cooling have, in many cases, 

 been crystallised, so as to assume those remarkable varieties of the 

 columnar form, so conspicuously developed in the north of Ireland, 

 the Scottish Islands, and many other parts of the world. 



The Basaltic rocks are characterised by a dark colour and a 

 compact base of the mineral called Labradorite, including black 

 pyroxene, generally the magnetic oxide of iron, frequently 

 peridote, and sometimes crystallised feldspar, to which they owe 

 their porphyritic structure. 

 76 



