jR. A. Daly — Secondary Origin of Certain Granites. 211 



So far as such water determinations in the crystallized rock can 

 be considered as indicating a true condition of the magma 

 before solidification, the table implies that the compound 

 magma corresponding to the u intermediate rock" still held 

 the extra water of assimilation up to the moment of crystalliza- 

 tion ; and, secondly, that the well differentiated magma corre- 

 sponding to the soda granite had lost about half of the water 

 of assimilation before final solidification. It is, accordingly, 

 quite possible that this extra water which the upper acid zone 

 could not hold in permanent combination, has been responsible 

 for the unusual amount of external metamorphism in the sedi- 

 ments south of the Pigeon Point intrusive. Similar reasoning 

 may apply to the Sudbury example, but the required elaborate 

 chemical study of its more complex terranes has not yet been 

 made. 



In favor of this hypothesis is the fact that, so far as known 

 to the writer, differential contact metamorphism of the kind 

 here in discussion has never been described in connection with 

 a sill that does not also show evidence of strong internal assimi- 

 lation. 



Finally, there is no cause yet well determined why water or 

 other solvents should be systematically concentrated from the 

 original magma along the roof of an intrusive sill. Such con- 

 centration may, indeed, be the rule, but it has apparently not 

 been announced by any worker among the thousands of basic 

 sills described in geological literature. 



The conclusion seems justified that the special intensity of 

 the metamorphism on the upper contact of certain intrusive 

 bodies is probably not due to the special activity of solvents in 

 the original magma along that contact. The explanation 

 seems to lie partly in the different liability of the roof -rocks 

 and floor-rocks to metamorphic change, but yet more in the 

 metamorphic effects of water vapor set free in the digestion of 

 the invaded hydrous sediments. This water vapor may have 

 also assisted in the solvent work of the magma at the main 

 upper contact, and, finally, in increasing the fluidity of that 

 magma. 



Magmatic Differentiation. — The development of basic, 

 intermediate and acid zones in each of the sills is, thus, believed 

 to have been the result of the density stratification of the com- 

 pound masma of assimilation. The efficiency of differential 

 density in separating out lighter acid material from the 

 heavier basic, has been ably discussed and affirmed by Loew- 

 inson-Lessing. * It is unnecessary to recapitulate his argument, 

 witli which the present writer is in full accord. 

 *Op. cit., pp. 344-354. 



