756 DEPARTMENT OF THE INTERIOR 



2 GEORGE V., A. 1912 



in its favour 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 kilometres 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 kilometre in diameter can offer only about 1,000,000 

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

 cube were shattered into cubes 10 metres on the side and then engulfed, the 

 magma would carry on the work of solution on 600,000,000 square metres 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 difficulty of understanding how plutonic magma, 

 which is capable of intrusion, can become superheated more than two or three 

 hundred degrees Centigrade. 



Fourthly, the stoping hypothesis has the special advantage of providing a 

 mechanism of thorough agitation within a batholith. Strong stirring of the mass 

 is induced by the sinking of xenoliths and by the necessary rising of the 

 magma locally acidified by their solution. This agitation can explain the 

 marvelous homogeneity in each large batholith. It helps greatly to explain 

 the manifest evidences of magmatic differentiation within batholiths — splittings 

 and segregations that cannot be due to the slow process of molecular diffusion 

 or to mere thermal convection. The whole process of stoping and the rising of 

 syntectic magma tends to equalize the temperatures in the batholithic chamber 

 and thereby we can understand the even grain and rapid, nearly simultaneous 

 crystallization of a batholith throughout its visible depth. 



Fifthly, the engulfment of blocks of geosynclinal sediments enriches all 

 parts of the batholiths with water, chlorides, etc., which so greatly aid solution; 

 while, on the older view, these agents are confined to the uppermost part of the 

 chamber. 



Sixthly, as already noted, the cleansing of syntectic films from contact of 

 solid and liquid is much the more rapid and perfect according to the stoping 

 hypothesis, thus providing and renewing conditions for molecular lowering of 

 the fusion-point along contacts. 



In short, the newer view has the advantage of not only better explaining 



