304 



PRINCIPLES OF STRATIGRAPHY 



fication at any angle except parallelism. Dikes may be simple or 

 complex. In the latter case, they are multiple when successive in- 

 trusions of the same material occur in the same fissure; and com- 

 posite when showing successive injections of dififerent material in 

 the same fissure. Eruptive veins are injections into irregular branch- 

 ing cracks, while apophyses are tongues of igneous matter pro- 

 jecting irregularly into the rock from a larger intrusive mass. A 

 bysmalith (Iddings-17 i/o^") (Fig. 41) is an injected mass of 



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Fig. 42. Section of a typical volcanic neck. (After Geikie, Ancient Volcanoes 

 of Great Britain, Vol. II.) 



igneous rock filling a more or less circular cone or cylinder in 

 strata, and having the form of a plug. This plug may reach the 

 surface or may end against a domed roof of strata, simulating a 

 laccolith in appearance. It dififers from other masses of this type 

 in that the enclosing strata have been displaced by faulting, a solid 

 block of nearly horizontal strata being lifted at one time by the 

 force of the intrusions. The Holmes bysmalith of the Yellowstone 

 is a typical example. With laccoliths this type agrees in having a 

 more or less well-defined floor of other rock on which it rests. 

 Necks (Fig. 42) are the solid plugs of lava filling old volcanic 

 vents. They are generally regular in outline and section. Chono- 

 liths (Daly-y -.498) (Fig. 43) are the intrusive masses injected 

 into actual or potential cavities formed by dislocation of rock 

 formations such as is brought about during mountain building, and 

 not classifiable as dikes, sills, or laccoliths, bysmaliths, or necks. 

 They are of irregular form, and show a complex relation to the 

 invaded formation. In the process of injection they may crowd 

 aside and mash the country rock, intruding irregularly into all the 

 fissures thus formed. Of the intruded igneous masses this is per- 

 haps the commonest kind. 



The second group or that of the parallel hypabyssal igneous 

 masses comprises, I. Intrusive sheets, II. Laccoliths, and III. 

 Phacoliths. The first of these includes (a) sills, and (b) interfor- 



