558 GEOTECTONIC (STRUCTURAL) GEOLOGY. [Boox IY. 
Coarse pegmatite veins abounding in large plates of muscovite, 
black tourmaline, and quartz, with occasional crystals of beryl and 
other minerals, merge into the surrounding granite, which for a few 
inches along the contact has a foliated structure precisely resembling 
that of a fine gneiss. Possibly this foliation may indicate motion of 
the granite mass along the line of fissure, while the rock itself or the 
materials of the fissure were still capable of molecular rearrangement.’ 
§ 4. Necks. 
Under this term are included the filled-up pipes or funnels of 
former volcanic vents. Every series of volcanic sheets poured out at 
the surface must have been connected either with fissures or with 
orifices probably opened in lines of fissure. On the cessation of the 
eruptions, the orifices have remained filled with lava or with frag- 
mentary matter. But unless subsequent denudation has removed 
the overlying cone, a vent lies buried under the materials which 
came out of it. So extensive, however, has been the waste of the 
surface in many old volcanic regions that the vents have been laid 

oa LP 
Fig. 285.—DIAGRAM-SECTION TO SHOW THE STRUCTURE oF OLD VOLCANIC VENTS, AND 
HOW THEY MAY BE CONCEALED AND EXPOSED. 
1. Tuff cone with basalt plug still buried under sedimentary accumulations ; 2. Tuff 
cone and basalt plug partially exposed by denudation. 
‘bare. In Fig. 285, two volcanic funnels are represented, one of 
them still buried under overlying formations, the other partially 
exposed by denudation. Such accumulations of volcanic material in 
and around the pipes of eruption are known as Necks. ‘The study 
of them brings before us some of the more deep-seated phenomena 
of volcanic action that cannot usually be seen at a modern volcano. 
A neck is circular or elliptical in ground-plan, but occasionally 
more irregular and branching, and may vary in diameter from a few 
yards up to a mile, or even more (Fig. 286). It descends into the 
earth perpendicularly to the stratification of the formation to which 
it belongs. Should rocks originally horizontal be subsequently 
tilted, a neck associated with them would of course be thrown out of 
the vertical (Fig. 285). As a rule, however, the vertical descent of 
the necks into the earth’s crust has been comparatively little inter- 
fered with. In external form necks commonly rise as cones or dome- 
1 Sce pp. 307, 3138. : 

