CRYSTALLIZATION-DIFFERENTIATION IN MAGMAS 421 



As a result of continued warping banded structure has developed in 

 successively higher and higher horizons, broadly speaking, though 

 as the top was approached the conditions would vary considerably, 

 and it would seem likely that banding would not be produced as 

 readily. However this may be, we may consider that the stage of 

 possible development of banding has passed, and that the whole 

 mass is approaching complete crystallinity. The lower portions 

 pass through this stage somewhat sooner that the higher portions 

 and finally become completely crystalline, the liquid adjusting itself 

 continually to the composition of the crystals with which it is in 

 contact, until finally all the liquid is used up. In upper portions 

 some interstitial liquid still remains, and principally in those parts 

 from which olivine has been removed by gravity the interstitial 

 liquid becomes of a granitic nature. As the plagioclase crystals, 

 continually adjusting their composition, approach labradorite, 

 AbiAn2, the plagioclase content of the liquid approaches acid 

 andesine, and the actual proportion of the interstitial liquid becomes 

 rather small. This is the stage at which a lateral thrust acting on 

 the mass may produce a squeezing upward of residual liquid and a 

 packing together of the crystals after the manner discussed in 

 some detail on an earlier page. It may be noted that in the upper 

 portion of the mass the warping becomes more nearly a simple 

 compressive thrust resulting from the tendency toward the folding 

 in of the two hor^s of the crescent so that the squeezing out may be 

 regarded simply as a result of a continuance of the warping action 

 and as expressing its result at quite a late stage of crystallization. 

 There is even a very definite suggestion in the shape of the lopolith 

 as mapped of a departure from the simple crescentic form. This 

 departure is, as might be expected, localized where the red rock is 

 developed in force and is of the nature of a bulging of the roof 

 (Fig. 5). Add to this the fact that the plagioclase of the red rock is 

 on the average precisely the proper plagioclase for a liquid in 

 equilibrium with the general plagioclase crystals of the gabbro, 

 and it becomes rather obvious that one is, to say the least, hasty who 

 dismisses crystallization-differentiation as a possible explanation of 

 the gabbro, red-rock association. The red rock has, on the contrary, 

 all the earmarks of a crvstallization residuum that could have been 



