772 DEPARTMENT OF THE INTERIOR 



2 GEORGE V., A. 1912 



Origin of Basic Contact-shells. — The writer believes that the principle of 

 gravitative differentiation is destined to supplant more and more the principle of 

 diffusion in petrogenie theory. That, for example, basic contact-shells in intru- 

 sive bodies are due to the diffusion of f erro-magnesian and cafemic constituents 

 toward the contact surface (Ludwig-Soret principle), is generally not the best 

 explanation, is illustrated in the often quoted case of Square Butte, Montana.* 

 Pirsson now explains the alkaline (sodalite) syenite of the core of this laccolith 

 as derived from a basic magma by a combination of crystallization, convection 

 currents, and settling-out. Calculation shows that the original magma had a 

 composition like that of the leucite basalt which occurs as lava flows in the 

 region. Shonkinite forming the lower, thicker part of the laccolith is the com- 

 plementary product of the differentiation. The present writer is rather inclined 

 to the view that, in this case, the two complementary masses separated in the 

 liquid phase, rather than that the shonkinite represents sunken phenocrystic 

 material. Ready calculation shows that, within a still liquid laccolithic mass, 

 the possible differences of density induced by contact cooling are extremely 

 minute. The true convection-currents must therefore be very feeble; and the 

 period of their activity must be short. 



The view that this differentiation has been due to a kind of liquation, 

 accompanied by a gravitative separation of the heavier and lighter fractions, 

 does not involve such an unfavourable condition. The process may be summar- 

 ized as follows : A leucite-basalt magma was injected in a liquid state. On all 

 sides of the laccolith it froze quickly, giving a basic contact-shell. The interior 

 part, much longer fluid, was cooled until it reached the temperature of liquation 

 (just above the point of solidification), and the splitting took place. This 

 hypothesis implies that the basic rock at the roof had the composition of a 

 leucite basalt. But the roof and this upper basic layer have both been com- 

 pletely eroded away so that it is not possible to test the truth of the inference. 



The Shonkin Sag laccolith shows the same kind of differentiation.! In this 

 case the roof and upper basic shell are still preserved. Pirsson describes the 

 vertical section at the middle of the laccolith as follows: — 



Thickness in feet. 



a. Leucite-basalt porphyry 5 



b. Dense shonkinite ' 5 



c. Shonkinite 5-6 



d. Transition rock 3 



e. Syenite 25-30 



/. Transition rock 15 



g. Shonkinite 60-75 



h. Leucite-basalt porphyry 15 



Total 140 



The syenite forms only about one-nineteenth of the laccolith. The small 

 difference chemically between shonkinite and leucite basalt would make it 



* L. V. Pirsson, Bull. 237, U.S. Geol. Survey, 1905, pp. 53 and 189. 

 fL. V. Pirsson, ibid., p. 47 ff, 



