4o8 N. L. BOWEN 



component of this hydrostatic pressure on the liquid will aid in 

 emphasizing the up-arching of the beds of the roof. As the crystal 

 mesh progressively breaks down, and the horizontal dimensions are 

 shortened in consequence of closer packing of crystals, there will 

 be a flow of the interstitial liquid into the space developed by the 

 up-arching of the roof. All three actions — shortening of the lateral 

 dimensions by closer packing of crystals, up-arching of the roof, 

 and movement of the liquid — go on pari passu and are thoroughly 

 interdependent. When the crystals have become sufficiently 

 closely packed to withstand the thrust the whole action must 

 cease, and of course may cease sooner for other reasons. The result- 

 ant mass is therefore, in the simplified case we have assumed, a 

 laccolith showing marked differentiation, with a rock represented 

 by crystalline material separated from the original magma as a lower 

 differentiate, and as an upper differentiate material represented by 

 a late residual liquid (Fig. ih). The differentiation would be 

 markedly discontinuous, the later differentiate would have an 

 intrusive relation to the earlier in places, in other places there would 

 be a rather abrupt transition, and yet the relations would not indi- 

 cate successive intrusion in the ordinary sense of the term. In so 

 far as this action is incapable of occurring before a certain degree of 

 crystallinity has been attained, there should be a tendency toward 

 a fairly constant contrast between the two differentiates in the 

 matter of composition. On the other hand there is no necessity for 

 constant relative proportions in their amounts. Associated igneous 

 rocks when formed by this method should present, therefore, a 

 fairly constant hiatus as compared with the complete igneous series 

 that is capable of production under other circumstances. Any 

 mass, not necessarily of the simple shape we have assumed but 

 capable of being acted upon in the same general manner, should 

 give a closely related result, so that the general phenomenon might 

 well be of widespread occurrence. The gabbro-granophyre 

 association fulfils every requirement. 



An example of the extreme simplicity we have pictured will of 

 course never be found. One of the principal compHcations results 

 from the fact that the freezing of such a mass will take place from 

 the outer layers inward, except for such modification of this behavior 



