32 REGINALD A. DALY 
the total amount of desilication observed in any nephelitic or 
leucitic body yet discovered. Considering the fluxing power of 
limestone or dolomite on silicate melts, such moderate, local assimi- 
lation is surely not improbable. More:siliceous sediments or even 
gneissic or granitic rocks may be simultaneously dissolved; but, 
on account of its character as a flux, a carbonate rock is likely to 
be absorbed in greater volume. Hence it does not follow that the 
solution of “‘an occasional bed of limestone in a terrane consisting 
principally of siliceous gneisses . . . . would entail simultaneous 
absorption of a much greater quantity of relatively siliceous 
material”’ (Bowen, p. 63). The relative amounts absorbed must 
really depend on a number of factors, including the contact relations 
of each layer of the country rocks to the invading magma. 
Meaning of melilitic. rocks.—Bowen’s admission regarding the 
melilite rocks is of particular significance when one remembers. 
the exceeding intimacy of alkali-rich rocks with melilite basalt, 
nephelite-melilite basalt, and alndite.* 
Résumé.—The prominent difficulties with Bowen’s theory may 
now be summarized. 
The sinking or rising of crystals in magmas is a true cause of 
diversity in igneous rocks, but it is not the only important cause; 
perhaps it is much less important than the separation of liquid 
phases. Apart from the possible development of ‘liquid immis- 
cibility”’ in an initially homogeneous magma, separation in the 
liquid phase is to be expected: (1) if the magma at the time of 
emplacement in a large chamber were heterogeneous; (2) if for 
any reason gases are concentrated locally within the magmatic 
body; and (3) if the assimilation of country rocks, or of their 
volatile constituents alone, takes place. The field evidences for im- 
portant assimilation are not consonant with the pure-fractionation 
theory. 
Bowen’s theory seems to imply a greater facility of differ- 
entiation for basaltic and other magmas than they actually possess; 
most sills and laccoliths are not visibly differentiated, even though 
they show features implying fluency after injection. The theory 
gives no good explanation of the comparatively abrupt transition 
« See the writer’s Igneous Rocks and Their Origin, p. 436, and Appendix D. 
