LATER STAGES OF EVOLUTION OF IGNEOUS ROCKS 9 



hensive variety of silicate mixtures, ideal conditions are maintained 

 for the discovery of liquid immiscibility. The charges are held at 

 various temperatures, both above the temperature of beginning 

 of crystallization and throughout its range, for a period of time 

 sufficient to obtain equilibrium between the liquid phase and the 

 crystalline phases. How much more readily would equilibrium 

 have been obtained between two liquid phases if there had been 

 any tendency toward the formation of two such phases ! Yet not a 

 single instance of liquid immiscibility has been encountered in the 

 whole course of this work. The experiments have been conducted 

 both at atmospheric pressure and in some cases in bombs under 

 high pressure of water-vapor. 



It may be pointed out that, for the detection of immiscibility, 

 it would not have been necessary to allow sufficient time for the 

 collection of the immiscible portions into separate layers. The 

 separation of globules of liquid o.oimm. in diameter and even 

 much smaller could not have escaped detection during the micro- 

 scopic examination of the quenched products. Continued experi- 

 mental work is, then, in accord with Vogt's conclusion, and it is safe 

 to repeat that the experimental evidence, as far as it goes, is decid- 

 edly against liquid immiscibility. 



In the case of natural rocks, differences of composition in 

 various parts of a body, when completely crystalline, might be 

 referred equally well to the separation either of crystalline or of 

 liquid phases. Similarly, there is no compelHng reason for pre- 

 ferring liquid immiscibility as the explanation of the association, 

 however intimate, of distinct lava flows of different composition, 

 such as the basaltic and rhyoHtic lavas of Iceland. Granting 

 that both came from the same source, it is possible that the extrusion 

 of the different varieties was separated by periods of years, during 

 which interval thpre was plenty of opportunity for a change in the 

 composition of the liquid in the magma basin due to the separation 

 of crystals from it. 



Even the cases, sometimes described, of simultaneous intrusion 

 or extrusion at the same locality of different magmas does not prove 

 their coexistence as immiscible liquids in the same pool. There is 

 often, on the contrary, distinct evidence, when two magmas are 



