494 REGINALD A. DALY 



greatly thickened sill of compact, domical form, with (a) a limital 

 thickness, (b) a limital area, (c) a flat base, (d) an oval or circular 

 ground plan, and (e) a specialized method of intrusion, namely, by 

 lifting a stratified cover which thus assumes a dome structure. 



Gilbert notes, as a first variation on the simple type, a compound 



type of laccolith which is "built up of distinct layers It is 



probable that all the larger laccolites are composite, having been 

 built up by the accession of a number of distinct intrusions." 1 



He continues: 



If the strata had experienced anterior displacements so as to be inclined, 

 folded, and faulted, a symmetrical growth of laccolites would have been impos- 

 sible, and the mountains would not have yielded a knowledge of the type form. 

 But the type form being known, it is to be anticipated that in disturbed regions 

 aberrant forms will be recognized and referred to the type. 2 



II. Cross 3 illustrates many examples of true laccoliths which 

 show aberrant forms in just such a way as was foretold by Gilbert. 

 Cross still holds that the body is a laccolith even if its expansion has 

 taken place from a plane only approximately parallel to the bedding 

 of the invaded strata. He emphasizes the asymmetric dome as more 

 nearly the real shape of a laccolith in nature, and attributes such 

 irregularity of form chiefly to lines of weakness existing in the sedi- 

 mentary formation before intrusion took place. A more fundamental 

 difference between the definitions of Gilbert and Cross appears in 

 their respective statements as to the method of intrusion. Cross 

 holds that the deformation of the stratified cover is in many cases not 

 simply due to the force of a gigantic hydrostatic press; that the 

 deformation was then incidental to the gaping of strata undergoing 

 lateral, orogenic pressure, the magma being more or less passive as 

 it was injected. 



III. Weed and Pirsson 4 agree with the views of Cross, but Pirsson 

 returns to the idea of the hydrostatic press as explanatory of the 

 intrusions in the Judith Mountains. Both authors consider that a 

 horizontal base is not necessary for a true laccolith, and that some 

 laccoliths are doubly convex. 



1 Op. cit., p. 55. 2 Ibid., p. 98. 



3 Fourteenth Annual Report, U. S. Geological Survey, Part 2 (1894), pp. 184 ff. 



4 Eighteenth Annual Report, U. S. Geological Survey, Part 3 (1898), p. 58.1. 



