THE BEHAVIOR OF INCLUSIONS IN IGNEOUS MAGMAS 559 

 THE ACTION OF BASIC MAGMAS ON SILICEOUS SEDIMENTS 



A number of examples of argillaceous quartzites are known that 

 have been invaded by basaltic magma and in which it is believed by 

 some investigators that absorption of the siliceous sediments by 

 the magma has occurred. Two carefully studied examples are the 

 Pigeon Point sill of Minnesota^ and the Moyie sills of British Colum- 

 bia.^ Such quartzites may again be regarded as consisting of 

 material belonging to a later stage of the reaction series than basaltic 

 magma, together with a certain surplus. The former, corresponding 

 in composition with some acid igneous rock, should be capable of 

 becoming part of the liquid magma by precipitating its heat equiva- 

 lent of the phases with which the magma is saturated. There is no 

 theoretical objection, therefore, to the belief that a certain amount 

 of the inclusions could be incorporated in this manner even though 

 the magma is saturated. It should be borne in mind, however, that 

 the material that can thus become a part of the liquid must be of a 

 composition toward which the magma could change spontaneously 

 by fractional crystallization. Once incorporated, it requires, to 

 produce the acid differentiates that were there formed, the same con- 

 ditions of crystallization as would have produced an acid differen- 

 tiate from the uncontaminated magma. In all of these examples the 

 normal course of differentiation is the primary consideration. The 

 extent to which incorporated material contributed to the bulk of 

 the acid differentiate may not have been important in these small 

 bodies even though there is plain evidence of incorporation. 



At both localities mentioned evidence of some incorporation is 

 unquestionable. About inclusions of the sedimentary rocks reaction 

 rims of a granitic nature have been formed. We have seen on page 

 529 that the reaction should emphasize, in the liquid around the 

 inclusion, material belonging to a later stage of the reaction series 

 (i.e., toward which the magma can crystallize), and this should not 

 be intermediate in composition between magma and inclusion. Cor- 

 responding with this deduction we find that the rims about the xeno- 



' W. S. Bayley, U.S. Geol. Survey, Bull. log. R. A. Daly, Amer. Jour. Sci., 

 Vol. XLIII (1917), p. 423. 



^ R. A. Daly, Geol. Survey Can., Mem. j8, p. 226. S. J. Schofield, Geol. Survey 

 Can., Museum Bull. No. 2. 



