REPORT OF THE CHIEF ASTRONOMER 719 



SESSIONAL PAPER No. 25a 



convex shape of the laccolith, but presents typically a meniscus, or some- 

 times a doubly convex form. Except where the folding has the character 

 of a dome, a phacolite does not show the nearly circular ground-plan of a 

 laccolith, but has a long diameter in the direction of the axes of folding. 

 As regards the mechanical conditions of its injection, the phacolite 

 resembles rather the small subsidiary intrusions which sometimes accom- 

 pany a laccolith, and are consequences of the sharp flexure caused by the 

 primary intrusion.'* 



It may be noted that ' phacolith ' is a preferable spelling because it is of 

 the same form as the internationally spelled 'laccolith,' ' batholith,' etc., all of 

 which have come directly from the Greek, and should therefore have the ' lith ' 

 termination. Moreover, the word ' phacolite ' is already pre-empted by miner- 

 alogy as the name of a zeolite. 



Some of the laccoliths as understood and described by Cross, Jaggar, and 

 others belong in the class of ' phacoliths.' 



Bysmalith. — Allied to ' plugs ' in Russell's sense is the 'bysmalith ' of 

 Iddings, described as an injected body filling a ' more or less circular cone or 

 cylinder of strata, having the form of a plug, which might be driven out at 

 the surface of the earth, or might terminate in a dome of strata resembling the 

 dome over a laccolith. The downward termination of the original type bysma- 

 lith (Mt. Holmes) is found in a hypothetical Archean floor on which the por- 

 phyry of the bysmalith rests. Such a body may be regarded as a laccolith which 

 lifted its cover along circumferential faults in the roof, rather than by the 

 mere flexing of its strata.f 



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

 sive with reference to the formations traversed by the lava, whether those for- 

 mations 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. 



Chonolith. — There remains for distinction a class of injected igneous 

 bodies which are not included in any of the above-mentioned categories. In the 

 dislocation of rock formations such as is brought about during mountain- 

 building, actual or potential cavities are formed within the earth's crust. 

 These are commonly filled with igneous magma squeezed into the individual 

 cavity from below, from the side, or, it may be, from above. Dikes, sills, and 

 bodies of laccolithic form (though not strictly of the laccolithic mode of intru- 

 sion, 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 for- 

 mations so complicated, that the body cannot be classified in any of the divi- 

 sions so far named. Again, irregular injected bodies of a similarly indefinite 

 va/iety or form are due to the active crowding-aside and mashing of the coun- 

 try-rock which is forced asunder by the magma under pressure. Or, thirdly. 



* A. Harker, The Natural History of the Igneous Kocks, New York, 1909, p. 77. 

 t J. P. Iddings, Monograph 32, Part 2, U.S. Geol. Survey, 1899, p. 16. 



