CRYSTALLIZATION-DIFFERENTIATION IN MAGMAS 419 



that the strong segment and indeed the whole crystal mesh which 

 we are considering is a thoroughly porous material permitting free 

 passage of the liquid residual magma under its hydrostatic head. 

 A space between the two layers could therefore develop freely, for 

 it would fill with liquid as fast as it opened. The yielding of the 

 weak segment, the opening of a long, narrow space between the 

 bridgelike strong segment and the lower layer, and the filling of 

 this space with the residual liquid would therefore go on simultane- 

 ously and indeed would be thoroughly interdependent. The 

 yielding of the weak segment would be the result of the breaking of 

 the relatively weak crystal mesh at the points where crystals had 

 begun to grow together, and once instituted it would continue until 

 the crystals were packed together rather closely with only a little 

 residual liquid, or until the mass had acquired a strength equal to 

 that of the segment that was acting upon it as a plunger. At this 

 time the action would cease as far as the yielding of that particular 

 weak segment was concerned. As a result then of this single 

 episode two layers have been produced whose direction is practically 

 parallel to the base of the igneous mass, one of them having a 

 composition practically equivalent to that of the residual liquid 

 contained in the crystal mesh at the time, and the other a composi- 

 tion not far from that of the crystals that had separated at the 

 time (Fig. 46). If we consider the further history of this par- 

 ticular part of the igneous mass as cooling and further bending 

 proceed it will be found that the segment that had yielded will, 

 when a certain amount of growing together of the already compacted 

 crystals takes place, constitute a particularly strong segment and 

 in virtue of its strength may be the means of production of a 

 layer of the same type as itself, from a relatively weak segment 

 sensibly along its strike, by acting as a plunger and compacting 

 that segment. At the same time the plunger segment will produce 

 immediately beneath itself a band of the opposite type. It is 

 thus seen that the action, once begun, is a means of its own growth 

 and is particularly competent to produce adjacent strongly con- 

 trasted bands. Constant repetition of this action as cooling and 

 warping proceed should produce innumerable bands such as those 

 seen in the Duluth mass. 



