CLASSIFICATION OF IGNEOUS INTRUSIVE BODIES 493 



Laccolith. — Divergence of definition and usage becomes very- 

 marked in the case of the term "laccolith." 

 I. The original definition by G. K. Gilbert : 



The station of the laccolite being decided, the first step in its formation is 

 the intrusion along a parting of strata, of a thin sheet of lava, which spreads until 

 it has an area adequate, on the principle of the hydrostatic press, to the deforma- 

 tion of the covering strata. The spreading sheet always extends itself in the direc- 

 tion of least resistance, and, if the resistances are equal on all sides, takes a 

 circular form. So soon as the lava can uparch the strata, it does so, and the sheet 

 becomes a laccolite. With the continued addition of lava, the laccolite grows in 

 height and width, until finally the supply of material or the propelling force so far 

 diminishes that the lava clogs by congelation in its conduit and the inflow stops. 1 



W. by N. 



E. by S. 



Fig. 



As a rule, laccolites are compact in form. The base, which in eleven localities 

 was seen in section, was found flat, except where it copied the curvature of some 

 inferior arch. Wherever the ground plan could be observed, it was found to be 

 a short oval, the ratio of the two diameters not exceeding that of three to two. 

 Where the profile could be observed, it was usually found to be a simple curve, 

 convex upward, but in a few cases, and especially in that of the Marvine laccolite, 

 the upper surface undulates. The height is never more than one-third of the 

 width, but is frequently much less, and the average ratio of all the measurements 

 I am able to combine is one to seven. 



The ground plan approximates a circle, and the type form is probably a 

 solid of revolution — such as the half of an oblate spheroid. 2 



The laccolite is a greatly thickened sheet [sill] and the sheet [sill] is a broad, 

 thin, attenuated laccolite. 3 



The laccolite in its formation is constantly solving a problem of "least force," 

 and its form is a result A laccolite grows "by lifting its cover." 4 



The clearness and precision of Gilbert's description gives a dis- 

 tinct and unmistakable individuality to his original type. It is a 



1 Op. cit. (1877), p. 95. 2 Ibid., p. 55. 3 Ibid.,' p. 20. 4 Ibid., p. 91. 



