REPORT OF THE CHIEF ASTRONOMER 251 



SESSIONAL PAPER No. 25a 



Likewise significant is the analogy of the Moyie sills to several igneous 

 masse- which have cut thick limestones and have then undergone differentiation 

 hy gravity. 



The well known laccoliths of Square Butte and Shonkin Sag, in Montana. 

 have been ably described by Weed and Pirsson.* In a later, independent publi- 

 cation, Pirsson has described the differentiation as due to the combined effect 

 of fractional crystallization, convection currents, and gravity.f In the present 

 writer's opinion, thermal convection must be of infinitesimal strength in such 

 bodies and he cannot find adequate explanation of the shonkinite and other 

 basic phases of these sills in fractional crystallization. On the other hand, 

 the writer finds most satisfaction in the view that the leucite-basalt porphyry 

 of Shonkin Sag, occurring at top and bottom of 'the laccolith, represents the 

 quickly chilled magma originally injected into the chamber. The syenite and 

 the shonkinite are the two poles of a gravitative differentiation of the remaining 

 leucite-basalt magma, which, in the heart of the mass, remained fluid long 

 enough for splitting. The segregation of the two polar magmas is of a kind 

 suggesting limited miseibility between them. The leucite-basalt can be 

 explained as itself a differentiate from basaltic magma which had dissolved 

 a moderate amount of the thick pre-Tertiary limestones traversed by the mag- 

 matic feeder of this laccolith. A similar explanation may be applied to Square 

 Butte. 



Tyrrell has described another noteworthy analogy in the I/ugar sill of 

 Scotland, where the alkaline pole is teschenite overlying the femic pole, a 

 picrite. This sill is injected into, the Millstone Grit; its feeder doubtless tra- 

 versed the underlying Carboniferous limestones and perhaps absorbed them 

 in some measure. Tyrrell explains the differentiation of the Lugar sill in 

 essentially the same way as that outlined by the present writer for the Shonkin 

 Sag laccolith. :{: 



Shand has recently described a large laccolith near Loch Borolan, Scotland, 

 in which quartz syenite (specific gravity 2-635) overlies quartz-free syenite 

 (specific gravity 2-65), which in turn overlies nephelite syenite (specific 

 gravity 2-67), and ' ledmorite ' (specific gravity 2-74 — 2-78). This mass 

 clearly cuts thick Cambrian limestone and other sediments. Shand attri- 

 butes the layered condition of the laccolith to differentiation under gravity. § 

 He makes no statement as to the origin of the magma thus differentiated. 

 On account of its ' desilicated ' character, the present writer is inclined to sus- 

 pect its derivation from a basalt-limestone syntectic. 



Pinally, the thick sill described by Noble as cutting the shales in the 

 Colorado canyon is worthy of special emphasis in the present connection." "' The 



* W. H. Weed and L. V. Pirsson, American Journal of Science, Vol. 11, 1901, p. 1. 

 and Fort Benton Folio, U.S. Geological Survey, 1899. 



tL. V. Pirsson, Bulletin 237, U. S. Geological Survey, 1905, p. 42. 



I G. W. Tyrrell, Transactions of the Geol. Society of Glasgow. Vol. 13, part 3, 

 1909, p. 298. 



5 S. J. Shand, Transactions of the Geol. Society of Edinburgh, Vol. 9, 1910, p. 376. 



** L. F. Noble, American Journal of Scieuce, Vol. 29. 1910, p. 517, 



