144 



GEOLOGY. 



have been more perfectly isolated and are most conspicuous. Most 

 of the even-crested ridges of the Appalachian system, as well as many 



Figs. 125-27. — Diagrams illustrating piracy, where the stream which does not flow 

 over rock of superior hardness captures those which do. Fig, 126 represents a 

 further development of the drainage shown in Fig. 125, and Fig. 127 represents a 

 still later stage. 



others which might be mentioned, became ridges in this way. In 

 the final stages of an erosion cycle the ridges of hard rock are themselves 

 brought low. Isolated remnants of hard rock which remain distinctly 



