718 DEPARTMENT OF THE INTERIOR 



2 GEORGE V., A. 1912 



broader definition, are agreed on the following characteristics : (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 injec- 

 tion 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 

 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 laccoliths, see 

 the writings of Gilbert, Cross, Weed and Pirsson, and Jaggar. 



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

 analogy of ' multiple dike ' and ' multiple sill.' It would differ from a com- 

 pound 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. 



Harker* has noted the occurrence of ' composite laccoliths ' in the island of 

 Skye. Through the chemical contrasts of their successively injected parts, they 

 are distinguished from multiple laccoliths. 



Weed and Pirssonf have described as a laccolith a great, lenticular mass of 

 porphyry injected along a surface of unconformity, namely, that between pre- 

 Cambrian crystalline 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 satisfactory name 

 ' interformational laccolith' for this case. (Compare a similar section of an 

 occurrence in the Black Hills of South Dakota, published in the Annals of the 

 New York Academy of Science, Vol. 12, 1899, p. 212.) 



Phacolith. — Harker has recently proposed ' phacolite ' as a new name for a 

 type among the ' concordant intrusions.' He writes : — 



' In the ideal case of a system of undulatory folds there is increased 

 pressure and compression in the middle limbs of the folds, but in the crests 

 and troughs a relief of pressure and a certain tendency to opening of the 

 bedding-surfaces. A concurrent influx of molten magma will therefore 

 find its way along the crests and troughs of the wave-like folds. Intrusive 

 bodies corresponding more or less closely with this ideal case are common 

 in folded districts. Since some distinctive name seems to be needed, we' 

 may call them phacolite^. The name laccolith has often been extended to 

 include such bodies, but this is to confuse together two things radically 

 different. The intrusions now considered are not, like true laccoliths, the 

 cause of the attendant folding, but rather a consequence of it. The situa- 

 tion, habit, magnitude, and form of the phacolite are all determined by the 

 circumstances of the folding itself. In cross-section it has not the plano- 



* A. Harker, Tertiary Igneous Rocks of Skye, 1904, p. 209. 

 t Journal of Geology, Vol. 4, 1896, p. 402. 



