CRYSTALLIZATION-DIFFERENTIATION IN MAGMAS 407 



exhibited by the sand is but roughly analogous to the phenomena 

 possible in a partly crystallized magma but may serve as a starting- 

 point in the discussion. The relative movement of liquid with 

 respect to crystals by this general method is probably not normally 

 possible with rock materials before the crystals have grown together 

 into a mesh. In order to discuss the action here suggested we shall 

 assume specific cases and consider, as well as may be, the mechanics 

 of the process. 



Laccolith with upper late differentiate.— Assume that a sheetlike 

 mass of magma is injected between two members of a sedimentary 

 series, with sHght arching of the upper member so that a fiat 

 laccolithic mass is produced. The fact 

 that the sheet was injected at that 

 horizon may be regarded as being prob- 

 ably determined by the competency of 

 the member immediately above it. If 

 this terrane is being acted upon by a 

 compressive thrust the tendency will 

 be to shorten the lateral dimensions, 

 and the competent member will yield 

 by forming a more pronounced arch, 

 while the relatively incompetent beds 

 immediately beneath the mass will 

 yield by shortening and thickening. 



This action, going on slowly, will result in a thickening and 

 shortening of the laccolithic mass, which offers no resistance until 

 crystallization has proceeded to such an extent that the crystals 

 touch each other and have grown together into a strong frame- 

 work. When sufficiently strong this crystal mesh may finally be 

 able to sustain the thrust and cause a pause in the shortening action. 

 The condition then existing might be represented by Fig. 2a. 

 The stress may accumulate until finally the crystal mesh begins to 

 break down, whereupon the beds beneath will yield as before, and 

 the competent beds above wUl bend as before. Likewise, coin- 

 cident with the beginning of the breakdown of the crystal mesh 

 the interstitial liquid will feel the pressure of the thrust and will 

 transmit it hydrostatically to all parts of the mass, and the vertical 



Fig. 2. — (a) Laccolith at an 

 advanced stage of crystallization ; 

 (b) same after being subjected to 

 lateral pressure at this stage. 

 Shows squeezing of residual 

 liquid (late differentiate) into 

 the crown of the arch. 



