160 OUTLINES OF FIELD-GEOLOGY PART i 



of their course. Every intcrstratificd sheet must have 

 been connected somewhere underneath with the intrusive 

 pipe or vein by which it rose to the surface, although the 

 connection may have been subsequently destroyed or con- 

 cealed. An intrusive mass, on the other hand, may never 

 have been connected with the surface at all. Interstrati- 

 fied igneous rocks prove the former existence of active vol- 

 canic vents at or near the localities in which they occur. 

 Intrusive igneous rocks may be due to ancient deep-seated 

 movements in the crust of the earth, which never gave 

 rise to any of those surface manifestations usually held 

 to be expressed by the term volcanic An accurate dis- 

 crimination between these two groups is of importance 

 when the history of a volcanic district has to be made 

 out. The field-geologist should especially bear in mind 

 the following leading characters of each of the groups. 



1. Intrusive Rocks may occur in the form of (i) 

 Veins (Figs. 55, 58, 63), traversing at any angle the 

 rocks among which they rise, (2) Vertical wall-like masses 

 or Dykes, (3) Irregularly circular masses forming the 

 upper ends of vertical columns or pipes called "Necks" 

 (Figs. 59, 60, 61), (4) Injected Beds, Sheets, or Sills (Figs. 

 55, 56), or (5) Irregular amorphous Bosses (Fig. 62). 



When an eruptive rock can be seen to intersect any 

 of the beds of a series of strata, its intrusive character 

 becomes at once apparent. But when it lies between 

 stratified rocks, and assumes the form of a bed, some care 

 is needed to make its intrusive character certain, for it 

 might then be taken for an interstratified sheet. It is 

 usually characterised by being much closer in grain near 

 its junction with the other rocks than in the central parts 



