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MP. G, W. TYEKELL OK THE 



[vol. lxxii, 



support the view that the gravitational sinking, either of immiscible 

 fractions, or of crystals, was the chief factor concerned in their 

 differentiation. In any case the details of the process are not 

 open to direct inspection, as the magma has been moved since 

 differentiation, and we are, therefore, limited to indirect inference 

 as to its nature. But the gravitational factor receives further 

 support from the phenomena directly observable in the central 

 ultrabasic stratum of the sill, where a gravitational differentiation 

 occurred subsequent to intrusion. This may be regarded as an 

 indication of the process Avhich took place, one stage previously, 

 in the larger chamber whence the Lugar sill was immediately 

 derived. 



During the progress of this work I favoured the view that 

 the units of differentiation were immiscible fractions of water- 

 rich teschenite, and comparatively anhydrous picrite, respectively. 

 The sharp interior contact between the upper teschenite and the 

 underlying theralite was regarded as analogous to the sharp 

 contact-plane that is developed between immiscible fractions, such 

 as aniline and water, or various mixtures of metals, which have 

 arranged themselves in order of density. It also became necessary 

 to assume that the sill was the product of a single act of intrusion, 

 and that it was heterogeneous at the time of intrusion. In order to 

 account for the dissimilarity between the teschenitic contact-facies 

 and the bulk-composition of the sill as a whole, the differentiated 

 sill was believed to have received an onward impulse, which pushed 

 it forward into cold rocks, leaving behind its original picritic or 

 theralitic contacts, and establishing new contacts with the succeed- 

 ing layers of teschenite. 



The recent work of Dr. Bow T en has discredited liquation theories 

 of differentiation, and has emphasized the importance of crystalliza- 

 tion-differentiation, where the crystals are continually removed, 

 either by sinking or zoning, from contact with the liquid in which 

 they were formed. 1 This process receives convincing support from 

 the results of experimental work upon silicate magmas, which has 

 now been carried on for many years in the Geophysical Laboratory 

 at Washington. I am now inclined to ascribe the main differen- 

 tiation at Lugar to the sinking of heavy crystals in a teschenitic 

 magma. This process is dealt with in greater detail in the next 

 section. 



The hypothesis of successive intrusion removes many of the 

 difficulties, and makes unnecessary many of the assumptions, 

 mentioned above. On this hypothesis the teschenite was intruded 

 first into cold rocks, forming fine-grained basaltic facies at 

 both margins. While it was still cooling, but probably solid, a 

 thick mass of picrite magma was intruded along its centre-plane. 

 During its crystallization the ultrabasic layer became stratified 

 according to density mainly by the sinking of olivine-crystals, as 



1 ' The Later Stages in the Evolution of the Igneous Rocks ' Journ f Geol. 

 Chicago (1915) Suppl. pp. 1-91. 



