H. A. Daly — Mechanics of Igneous Intrusion. 39 



deep, to become convinced of the ultimate fate of that block. 

 If the somewhat cooled lavas described by Lacroix,* von John,f 

 Dannenberg,;}; Sandberger§ and others could dissolve rock- 

 inclusions in the notable way described by those authors, we 

 must credit a vast solution al efficiency to plutonic magma 

 when it attacks similar blocks in great depth. The lava has a 

 few T hours or days in which to do its work ; the abyssal magma 

 has centuries if not a large part of a geological period ! 



It must be remembered that geosynclinal sediments are 

 rocks unusually rich in water, chlorides, sulphur trioxide, etc. ; 

 all substances aiding solution in the primary magma and in 

 the secondary ( syntectic ) magma itself. It is probably also 

 owing to these fluids in large part that granitic magmas have 

 crystallized at comparatively low temperatures. 



The conception of stoping with abyssal assimilation has many 

 more points in its favor than can be cited for pure marginal 

 assimilation. A few of the special grounds for preferring 

 the newer to the older hypothesis may be noted. 



First, marginal assimilation is largely effective only in the 

 earliest part of the magma's history, when it is absolutely and 

 relatively very hot. There is thus an early time-limit fixed 

 for the gigantic work of dissolving the thousands of cubic kilo- 

 meters actually replaced in the intrusion of a large batholith. 



Secondly, the assimilation, on the older view, takes place 

 primarily on main contacts and along a relatively limited 

 amount of surface. For example, a cube of wall-rock one kilo- 

 meter in diameter can offer only about 1,000,000 square meters 

 of surface at a time to the dissolving magma. If that same 

 cube were shattered into cubes 10 meters on the side and then 

 engulfed, the magma would carry on the work of solution on 

 600,000,000 square meters of surface. 



Thirdly, the average crust-rock being allied chemically to 

 gneiss, is more soluble in basic magma than in acid. On the 

 stoping hypothesis, solution of the xenolith generally occurs in 

 the lower, basic part of the magmatic chamber; on the older 

 view, it is granitic magma which must do most of the work of 

 solution. For even if the originally injected magma is a 

 basalt, the products of its assimilating activity, being more 

 acid and less dense than itself, must remain at the batholithic 

 roof and rapidly assume the chemical composition of mean 

 mountain-rock. It follows that the primary magma must be 

 enormously more superheated than is required on the stoping 

 hypothesis or than seems easy of explanation, in view of the 



* Les Enclaves des Koches Volcaiaiques, Macon, 1893. 

 \ Jahrb, d. k. k. Eeichsanstalt, Vienna, Hi, p. 141, 1902. 

 JTseherm. Min. u. Petrogr. Mitth., xiv, p. 17, 1895. 

 § Sitzungsber. K. Bair. Akad. Wiss., p. 172, 1872. 



