1 66 



OUTI.IM - 01 INI I>-(;EOLOGY 





been mapped in the course of tin ( ieological 

 Survey of Scotland, in the Old Red Sandstone, Ca 

 iferous, and Permian system. 



The deep seated or plutonic rocks frequently occur as 

 Bosses. ( Iranitc and other massive rocks usually assume 

 this form. They include no true fragmental series, nor 

 have they any cellular, scoriform portions, for these could 

 only be formed towards the surface, where by relief from 

 pressure, the imprisoned steam had liberty to expand 

 and push aside the particles of the still fluid rock. 



FIG. 61. Section through the same Neck as in Figs. 59 and 60. 



Having cooled slowly under great pressure, they are 

 often highly crystalline. They occur as huge amorphous 

 masses, sometimes seventy miles long, as in the granite 

 of the south-east of Ireland. They also assume the 

 form of veins and dykes, often ramifying in the most 

 complex way through the rocks which they traverse. A 

 ring or "aureole" of metamorphism is usually to be 

 recognised round large eruptive bosses of plutonic rocks. 

 Within this ring, which may vary from a few yards to a 

 mile or more in breadth, a series of interesting mineral- 

 ogical reconstructions may be found, and gradations 



