6 INTRODUCTION. 



one break, and from that finds its way into a second, 

 a third, and a fourth, so that the river's course below 

 the falls assumes an extremely acute S-shaped form. 

 At home it is illustrated in the ravines occupied by 

 the lower portion of the Suir, the Barrow, and the 

 Nore, in south-east Ireland, where the different 

 streams have excavations along different systems of 

 breaks ; while in general the breaks along whose 

 continuations those rivers originally worked are 

 now occupied by tributary streams. A remarkable 

 instance of how a river may leave its original joint- 

 line is seen at the junction of the Suir and the 

 Barrow. The former is the larger stream, yet it has 

 left the course it might have been naturally expected 

 to follow, and now flows in the continuation of the 

 break along which the Barrow came, previous to 

 their junction. Numerous other illustrations could 

 be given ; these details, however, are more suited to 

 the body of this book than to its introduction ; it is 

 therefore unnecessary to enter further into the sub- 

 ject. 



Although valleys seem not to be formed without 

 the assistance of breaks or faults, yet all fault-lines 

 do not form valleys ; on the contrary, they may form 

 peaks and ridges. Nearly all the peaked summits 

 among the mountains of West Galway belong to 

 hills formed of quartz schist, and these rocks are 

 traversed by large dykes of hard "fault-rock," a 



