496 



REGINALD A. DALY 



same kind of material. If the magma has all been intruded at 

 practically the same time, we have the "compound laccolith" of Weed 

 and Pirsson. 1 



A multiple laccolith may be conceived, the name being formed on 

 the analogy of "multiple dike" and "multiple sill." It would differ 

 from a compound laccolith only in the fact that the deformation of the 

 strata, while again similar in character to that produced during the 

 intrusion of a simple laccolith, has been due to distinctly successive 

 injections of the same kind of magma. This case has not yet been 

 described as actually occurring in nature. 



Fig. 4 



Composite laccolith. — Harker 2 has noted the occurrence of "com- 

 posite laccoliths" in the island of Skye. The analogy with composite 

 dike and sill is again perfect. The principal distinction from both 

 compound and multiple laccoliths is found in the heterogeneous 

 nature of the magma successively injected in this last case. (See 



Fig. 4-) 



Interformational laccolith. — Weed and Pirsson 3 have described 

 as a laccolith a great,. lenticular mass of porphyry injected along a 

 surface of unconformity, namely, that between pre-Cambrian crystal- 

 line schists and a sedimentary Cambrian formation. Such a type 

 is again aberrant from Gilbert's types, but should certainly be classed 

 among the laccoliths; the writer proposes the not altogether satis- 

 factory name "interformational laccolith" for this case. (See Fig. 5, 

 and compare a similar section of an occurrence in the Black Hills of 

 South Dakota, published in the Annals of the New York Academy 

 0} Sciences, Vol. XII (1899), p. 212.) 



1 Op. cit., p. 580; see figure. 



2 Op. cit., p. 209. 3 Journal o) Geology, Vol. IV (1896), p. 402. 



