CMAf. XII 



IGNEOUS ROCU 



163 



55, to send out ramification*, though this is not so 



common as might be supposed In some volcanic dit- 



., the molten material has been injected into innui 



able irregular rents, and has consolidated there as Veins. 

 The rocks underlying the Tertiary Volcanic plateaux of 



:i present hundreds of striking illustrations (Fig. 58). 



- amount of alteration on the surrounding rock pro- 

 duced by dykes is usually surprisingly small. Sandstone 

 is hardened into a kind of quartzite, and occasionally 

 even acquires a columnar structure, the prisms being 

 directed outwards from the sides of the dyke. But the 

 alteration seldom reaches beyond a few feet Shales suffer 

 more, as they are found baked into a kind of porcelain- 

 jasper; but this may frequently be observed not to extend 



