THE BEHAVIOR OF INCLUSIONS IN IGNEOUS MAGMAS 543 



The limiting factors are principally mechanical rather than thermal 

 or chemical and are very difficult to evaluate. It should be noted, 

 in particular, that the combination which is most favorable for sig- 

 nificant effects in the way of reactive solution, viz., decidedly acid 

 inclusions and decidedly basic magma, is unfavorable in another 

 respect. The inclusions will be lighter than the magma and will not 

 tend to sink in it, whereas it is the sinking of inclusions through the 

 magma which favors particularly notable reaction effects since it 

 continually brings new magma into contact with the inclusions. 



As an example of the effect of basic magma on more acid 

 igneous inclusions basaltic magma and granitic inclusions have 

 been taken. Between such extremes the more marked effects 

 should be obtained, but it cannot be doubled that any basic magma 

 can dissolve, by the same reactive process, inclusions of a rock later 

 in the reaction (crystallization) series. Direct melting of granitic 

 inclusions to masses of liquid by basaltic magma is not ordinarily 

 to be expected because the solid granite does not retain the volatile 

 components that aid in lowering the melting temperature of granitic 

 magma below that of basaltic magma. This lack is no bar to the 

 reactive solution process described, though it may limit it some- 

 what. 



EFFECTS OF MAGMA ON INCLUSIONS OF SEDIMENTARY ORIGIN 



The general problem of the effects of magma upon inclusions 

 of sedimentary origin is much more difficult than the similar prob- 

 lem in connection with igneous inclusions. Sedimentary rocks have 

 their compositions determined by processes wholly independent of 

 igneous action and do not correspond in composition with the prod- 

 ucts precipitated from magmas at any stage of their career, that 

 is, cannot be placed definitely in the reaction series. However, 

 certain minerals that can be formed in magmas do occur in the sedi- 

 mentary rocks and often the composition of a sediment is such that 

 by mere heating it can be transformed into an aggregate made up 

 exclusively or almost exclusively of igneous rock minerals. Again 

 sediments exhibit extremes of composition, being very rich in cal- 

 cium carbonate, aluminum silicate or silica itself, and these present 

 a special problem. Yet it is perhaps not generally realized how 



