36 PEOCEEDIXGS OF THE NATIONAL MUREUM. vol.66. 



continual enrichment of the residual (interstitial) liquid in albite. If it is cooled 

 still more slowly, sinking of plagioclases causes a similar continual enrichment 

 of the residual (upper) liquid in albite. In favorable cases the final liquid may 

 be more than 90 per cent albite even although the original mixture were, say, 50 

 per cent diopside and 50 per cent bj'townite. 



If we now combine the information furnished by the investigated systems, 

 important conclusions may be drawn with regard to the crystallization of basaltic 

 magma under various conditions. Instead of the simple pyroxene, diopside, 

 present in the mixtures of the last system discussed, we may consider one of the 

 intermediate pyroxenes, wdiich melt with decomposition and the formation of 

 olivine, to be present in addition to plagioclase. Rapid cooling of such a liquid 

 would give merely plagioclase and pyroxene. On the other hand, slow cooling 

 permits radical variation from this simple result. The early formation of olivine 

 brings about an excess of free silica in the residual liquid if any process intervenes 

 to prevent the resorption of olivine by the liquid. The early formation of very 

 calcic plagioclase brings about an enrichment of the liquid in albite if anything 

 i ntervenes to prevent the continual alteration in the composition of the crystals 

 by interchange with the liquid. Finally the early formation of magnesia-rich 

 pyroxene brings about an enrichment of the liquid in diopsidic pyroxene if simi- 

 lar conditions intervene. 



The sinking of crystals affords a means of continually separating crystals from 

 the part of the Hquid in which they formed and is therefore a process which will 

 give the results just outlined. If, therefore, the mixture of plagioclase and 

 pyroxene referred to were cooled slowly and continual sinking of crystals oc- 

 curred, the inevitable result would be a body consisting of calcic plagioclase, 

 olivine, and magnesian pyroxene in its lower parts (i. e., of a gabbroidal nature) 

 and of sodic plagioclase approaching albite, diopsidic pyroxene, and free silica 

 in its upper parts (i. e., of a granitic nature), with various intermediate types in 

 the intermediate layers. If the freedom of sinking of crystals were somewhat 

 restricted, one of these intermediate types, say a granodiorite or a diorite, would 

 occur as the uppermost differentiate, the limit of the process under these less 

 favorable conditions. The composition of the residual Hquid might, moreover 

 have been similarly affected by zoning of the crystals even if there were no oppor- 

 tunity for the sinking of crystals, and in this case the interstitial material of late 

 crystallization would be the same salic material as that found in the upper layers 

 where sinking of crystals took place. If a certain amount of both zoning and 

 sinking of crystals took place, a body would result showing the salic differentiate 

 both as interstitial material and as a separate upperlayer. * * * 



It has been possible, then, to deduce from facts ascertained experimentally the 

 crystalHzation with quick and slow cooling of mixtures which give results closely 

 analogous to the occurrence observed in nature of diabase in small dikes and 

 small sills (quickly cooled) and of diabase with micropegmatite interstices or a 

 granitic or granodioritic differentiate in larger bodies (slowly cooled). There 

 are many differences and complications in the natural magma in the matter of 

 details, but it is clear that the broad scheme is well understood and that crys- 

 tallization is the sole control. There is no necessity for assuming that assimila- 

 tion of siliceous material is essential to the formation of the salic differentiate, 

 nor that its separation is accomplished by the process of liquid immiscibility. 



It is necessary to consider all of the examples of differentiation 

 above described and the several explanations advanced to account 



