498 



REGINALD A. DALY 



Volcanic neck. — The solid-lava filling of a volcanic vent is evidently 

 intrusive with reference to the formations traversed by the lava, 

 whether those formations are composed of non-volcanic rocks or of 

 agglomerate or tuff which has been pierced by thoroughly molten 

 lava on its way to the surface. (See Fig. 6.) 



u Chonolith" — There remains for distinction a class of injected 

 igneous bodies which are not included in any of the above-men- 

 tioned categories. In the dislocation of rock formations such as is 

 brought about during mountain-building, actual or potential cavities 

 arejx>rmed within the earth's crust. These are commonly filled with 

 igneous magma squeezed into the individual cavity from below, from 



Fig. 6 



the side, or, it may be, from above. Dikes, sills, and bodies of lac- 

 colithic form (though not strictly of the laccolithic mode of intrusion, 

 as designated by Gilbert) may thus originate. Yet very often the 

 shape of the intruded mass is so irregular, and its relations to the 

 invaded formations so complicated, that the body cannot be classified 

 in any of the divisions so far named. Again, irregular injected bodies 

 of a similarly indefinite variety or form are due to the active crowding- 

 aside and mashing of the country-rock which is forced asunder by 

 the magma under pressure. Or, thirdly, such bodies may be due to 

 a combination of the two primary causes — orogenic stress opening 

 cavities, and hydrostatic or other pressure emanating from the magma 

 itself and widening the cavities. 



The number and total volume of these irregular intrusions doubt- 

 less greatly exceed the number and volume of all the true laccoliths 

 of the world. In the average mountain range the geologist is more 

 likely to encounter injected bodies of the former kind than he is to 

 discover true laccoliths. 



