XII 





61 



uass(Fig.57>. This "chilled edge "not only 

 to distinguish an intrusive from contemporaneously 

 interstratified sheet of igneous material, but may be used 

 to determine the relative age of intrusive masses which 

 are associated top A here one of these masses 



presents a chilled edge to another mass it may be 

 inferred to be the younger of the two. Again, the rocks 

 lying upon an intrusive rock may be hardened, and some- 

 times exceedingly altered, while detached portions of 



them are now and then found to have been caught up 

 and entangled in the crystalline mass below. 



The position of the prismatic joints by which volcanic 

 rocks are frequently transversed may sometimes suffice to 

 indicate whether a rock is certainly intrusive or possibly 

 interstratified. These joints start from the cooling sur- 

 faces of the original melted mass of lava. In a bed they 

 are of course perpendicular to its upper and under sur 



M 



