CLASSIFICATION OF IGNEOUS INTRUSIVE BODIES 495 



IV. Jaggar 1 found that the laccoliths of the Black Hills illustrated 

 conclusions essentially equivalent to those of Cross, states his belief 

 that the magmas were mostly passive during intrusion, the laccolith 

 chambers being opened by orogenic stresses, and remarks that lac- 

 coliths may be doubly convex. 



V. The recent definition of Chamberlin and Salisbury 2 returns 

 once more to the original type of Gilbert: "If, after rising to a cer- 

 tain point in the strata, the lava arches the beds above into a dome, 

 and forms a great lens-like or cistern-like mass, it constitutes a 

 laccolith." 



It is seen that there is considerable diversity of usage for the term 

 "laccolith." On the whole, this diversity is a sign of progress in 

 geological science. Gilbert's ideal type has been supplemented by 

 others that vary from the ideal in one or more particulars, under 

 conditions which Gilbert himself foretold, if it were but in brief 

 expression. 



Those who have made actual researches among laccoliths, and 

 have preserved the term "laccolith" with the original meaning of 

 Gilbert's broader definition, are agreed on the following character- 

 istics: (a) Whatever the origin of the force involved, a laccolith is 

 always injected, (b) A laccolith is always in sill-relation to the 

 invaded, stratified, formation; that is, the injection has, in the main, 

 followed a bedding-plane; but, like sills, laccoliths often locally 

 break across the bedding, (c) A laccolith has the shape of a plano- 

 convex or doubly-convex lens flattened in the plane of bedding of 

 the invaded formation. The lens may be symmetric or asymmetric 

 in profile, circular, oval, or irregular in ground plan, (d) There are 

 all transitions between sills and laccoliths. 



For many illustrations of simple symmetric and asymmetric lac- 

 coliths, see the cited writings of Gilbert, Cross, Weed and Pirsson, 

 and Jaggar. 



Compound laccolith. — In the Judith Mountain type, as in the larger 

 examples of laccoliths in the Henry Mountains, the whole intrusive 

 body is divided by strong beds of the invaded formation. This gives 

 the appearance of a number of distinct intrusions, one of them domi- 

 nating, the others subsidiary, in size, but all of them composed of the 



1 Op. tit., p. 173. 2 Geology, Vol. I, p. 476. 



