286 Pirsson and Bice — Geology of Tripyramid Mountain. 



best accords with the ascertained facts. It is, of course, possi- 

 ble that the monzonite is entirely surrounded by gabbro, but 

 we have no proof of this and have thus shown it only on the 

 two sides where the outcrops have been seen, or are reported. 

 The relative sizes of the different areas may vary considerably 

 from what we have shown, but this is a detail which does not 

 affect in principle the correct understanding of the structure 

 and petrologic relations of the complex. 



Concentrically arranged complexes of igneous rocks similar 

 to this have been described from numerous localities, and of 

 alkalic types. Confining examples to North America, they 

 have been shown to be common among the linear groups of 

 elevations running southwardly from Montreal and termed the 

 Monteregian Hills by Professor Adams, which have been 

 described by him and other Canadian geologists.* Washing- 

 ton has shown this arrangement exists at Magnet Cove, Arkan- 

 sas, and also in the complex in Essex County, Mass.f Other 

 examples have been described from several localities in 

 Montana.:}: Still others might be mentioned, but the dozen 

 here cited are sufficient to illustrate this mode of occurrence 

 and serve as a basis for discussion. 



On the grounds of the field evidence obtained the authors 

 cited have in some of these cases considered the occurrence as a 

 laccolith within which subsequent differentiation has produced 

 the concentric complex ; in others they have held them to be 

 stocks, or volcanic necks, in which differentiation has occurred, 

 sometimes with subsequent movement of the differentiated 

 bodies of magma. At Square Butte and the Shonkin Sag the 

 field evidence as to the laccolithic character of the intrusions 

 is ample and decisive, as it seems to be also that Mt. Johnson 

 is a volcanic neck and Yogo Peak an intrusive stock. In other 

 cases the evidence is less clear, and the nature of the intrusion 

 is largely inferred ; thus Harker§ questions Washington's 

 interpretation of Magnet Cove as a single differentiated lacco- 

 lith, and suggests the alternative view that it may consist of 

 two thin, superposed laccoliths of different composition, which 



* Adams, F. D., The Monteregian Hills, Jour. Geol.. xi, p. 239, 1903. 

 Dresser, T. A., Geology and Petrog. of Shefford Mt., Quebec, Ann. Rejj. 

 Canad. Geol. Surv., xiii, pt. L, 1902. Geology of Brome Mt., this Journal, 

 xvii, p. 347, 1904. Young, G. A., Geology and Petrog. of Mt. Yamaska, 

 Quebec ; Ann. Eep. Canad. Geol. Surv. , xvi, pt. H, 1906. 



f Washington. H. S., Igneous Complex at Magnet Cove, Bull. Geol. Soc. 

 Amer., xl, p. 389, 1900. Foyaite-Iolite Series of Magnet Cove, Jour. Geol., 

 ix, p. 607, 1901. Petrog. Province of Essex Co., Mass., ibid., viii, p. 473, 1899. 



JWeed and Pirsson, High wood Mts. of Montana, Geol. Soc. Bull. Amer., 

 vi, p. 400, 1895. Igneous Rocks of Yogo Peak, Montana, this Journal, 

 1, p. 467, 1895; 20th Ann. Rep. U. S. Geol. Surv., pt. Ill, p. 563, 1900. 

 Bearpaw Mts. of Montana, this Journal, i, p. 351, 1896. Shonkin Sag and 

 Palisade Butte Laccoliths, ibid., xii, p. 1, 1901. 



§Geol. Mag., Dec. iv, vol. ix, p. 177, 1902. 



