EFFECTS OF SOLUTION OF CARBONATES 109 



of geologists and others who, hy training, are better qualified to discuss 

 the physico-chemical processes involved. A few illustrative points may be 

 noted, with the commonest lava, basalt, taken as a type of the subalkaline 

 magma. 



1. Limestone or dolomite is a flux for basalt. The solution of a small 

 proportion of either carbonate promotes fluidity. If there be any ten- 

 dency toward the separation (by gravity) of leucocratic and melanocratic 

 elements, this difl'erentiation is directly aided by increased fluidity. 

 Square Butte and Shonkin Sag prove very clearly that there is such ten- 

 dency in magma (leucite basalt, cutting thick limestones) which, in 

 chemical composition, differs relatively little from feldspar basalt.* 



The facts concerning many other igneous bodies teach the same truth, 

 namely, that gravitative differentiation, with concentration of alkalies in 

 the upper part of magma chambers, is a normal process in those magmas 

 which retain considerable fluidity near the crystallization temperatures of 

 the femic and '^''cafemic" (calcium-iron-magnesium) components. This 

 result is evidently brought about by fluxing. 



2. The solution of limestone in basalt is accompanied by the dissocia- 

 tion of at least part of the carbonate. The lime binds several times its 

 own weight of silica, with which iron oxides and magnesia may be also 

 combined, forming pyroxene. The introduction of more of the ion, CaO, 

 already present in the magmatic solution, promotes the precipitation of 

 augite, anorthite, and other lime-bearing crystals or liquid phases.^ An 

 inoculation of the solution is evidently produced by the exotic carbonate. 



3. The sinking of the femic and cafemic constituents (in solid or 

 liquid phases) leaves the upper, residual part of the magma richer in 

 alkalies than the original basalt. So far as the foreign lime has bound 

 silica and carried it down in the sunken constituents, the residual magma 

 is desilicated as compared with the original basalt. If the desilication is 

 sufficient, nephelite forms instead of plagioclase. If augite is formed by 

 the interaction of the foreign rock, a considerable desilication is possible, 

 for the lime then binds about 2.5 times its own weight of silica. The 

 formation and sinking of calcic, cafemic, and femic constituents leave 

 the residual solution rich in alumina. 



4. The carbon dioxide freed from the lime of the foreign rock must 

 have a profoundly disturbing effect on the chemical equilibrium of the 

 basalt. What that effect is can not be told until new, complete experi- 



* L. V. Pirsson : Petrography and geology of the igneous rocks of the Highwood Moun- 

 tains, Montana, Bulletin 237, U. S. Geological Survey, 1905. 



^ Similarly, if barium chloride is added to a saturated solution of barium sulphate, 

 almost all of the barium sulphate is precipitated. See J. H. L. Vogt : Tschermak's 

 MIneralogische Mitteilungen, vol. 27, 1908, p. 134. 



