17K) GORGES AND RAVINES. 



to be filled by glaciers " still engaged in cutting into 

 the mountains like endless saws." 



Some such ravines may possibly be due in part to 

 shrinkage, as it is not uncommon to find between 

 the rotten granite and its walls a thick parting of 

 flucany stuff, or a thin rib, or even a good metal- 

 liferous vein ; any of which would be due to a shrink- 

 age fissure. These partings and ribs have been ob- 

 served along the walls of many of the courses of 

 " rotten granite " in West Galway, while some of 

 the metalliferous veins in Cornwall are connected 

 with courses of " soft growan ; " and from Wyley's 

 description of the mines in Namaqualand, South 

 Africa, it would appear that some of those lodes are 

 similarly circumstanced. 



The denuding out of dykes of fault-rock, and 

 consequent formation of gorges or narrow fissures, 

 have already been described, and we may now add 

 that dykes of igneous rocks, when weathered away, 

 may leave very similar passes. These, indeed, may 

 have been at first shrinkage fissures, which sub- 

 sequently were filled with the intrusive rocks ; as it 

 is not uncommon to find the rock on both sides of 

 a dyke unbroken or unbent, which could scarcely be 

 the case if the igneous rocks had forced open the 

 fissure they now occupy; besides, in some cases ribs, 

 and even thick mineral veins, occur alongside dykes 

 of igneous rocks, as if there had been a shrinkage 



