lo JOURNAL OF GEOLOGY— SUPPLEMENT 



brought together in this manner, that they have a strong disposition 

 to mix and do so to the extent of their ability before crystallization 

 or cooling to the glassy state ensues. The facts point rather to 

 derivation of these magmas in most cases from separate, though 

 adjacent, pools, say, laccolithic chambers/ The pools may origi- 

 nally have contained the same magma, but on account of a differ- 

 ence in size and consequently in rate of cooling were, at the time of 

 reintrusion, at different stages in their career of crystallization and 

 therefore, as will be shown in the sequel, the still liquid portion 

 drained-off from the one had a different composition from that 

 drained from the other. 



There is only one kind of phenomenon which could be con- 

 sidered as definite proof of the occurrence of limited liquid misci- 

 bility and it would, without doubt, be very commonly observed if 

 limited miscibility among rock-forming silicates were a fact. The 

 observed phenomenon would be the occurrence in glassy, or partly 

 glassy, extrusive lavas of distinct globules of material still partly 

 glass, large or small according to their opportunity for aggregation, 

 and of composition different from that of the main mass. Here 

 would lie indisputable proof of the formation of immiscible liquid 

 globules. So far as the writer is aware, no such case has been 

 described, in spite of the fact that lavas, as a group, depict aU 

 igneous processes arrested, more or less successfully, at all possible 

 stages. They are Nature's quenching experiments. 



The rare globular masses found in those completely crystalline 

 plutonic rocks to which the term orbicular has been applied are 

 an entirely different matter. It is often demonstrable that these 

 owe their origin to the localization of crystallization of certain 

 constituents, due to the cooling and chemical effect of a foreign 

 inclusion, the remains of which often constitute the nucleus of the 

 orbule.^ The occurrence of such structures as an exceedingly 

 rare curiosity shows that they always require special conditions and 

 are not a normal result of cooling as the formation of immiscible 



' Certain stages of crystallization at which the Uqnid portion may have different 

 compositions in different parts of a single reservoir will be discussed later. 



^ Von Chrustschoff lists other possibilities {Mem. Acad. Sci. St. Petershourg [7], 

 XLII, No. 3). 



