
a 

Parr VIL. Suor. i. § 3.] VEINS AND DYKES.’ on 
during the earlier stages of volcanic action before communication 
had been established with the surface, or towards the close when, the 
vents having been choked up with erupted material, escape to the 
surface became difficult. 
§3. Veins and Dykes. 
The term “vein” is rather vaguely employed by geologists. It 
is used as the designation of any mass of mineral matter which has 
solidified between the separated walls of a fissure. When this 
mineral matter has been deposited from aqueous solution or from 
sublimation, it forms what is known as a mineral vein (p. 589). When 
it has been injected in a molten or pasty state, it forms an eruptive 
vein ; or, if it forms a vertical wall-like mass, a dyke. When it has 
crystallized or segregated out of the component materials of some 
still unconsolidated, colloid, or pasty rock, it is called a segregation 
vein. 
Eruptive or Intrusive Veins and Dykes are portions of once- 
melted or at least pasty matter which have been injected into 















































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Hic. 277.—INTRUSIVE VEINS AND DYKES OF PORPHYRITE IN TUFF OF A VOLCANIC 
“ NEecK,” RENFREWSHIRE. 
rents of previously solidified rocks. When traceable sufficiently far, 
they may be seen to swell out and merge into their parent mass, 
while in the opposite direction they may become attenuated into 
mere threads. Sometimes they run for many yards in tolerably 
straight lines, and when this takes place along the stratification 
