Lawson.] 



Orbicular Gabbro. 



395 



hoped that the account given might prove a slight contribution 

 to our knowledge of this interesting class of rocks. But the 

 writer cannot close the paper without at least indicating that in 

 the orbicular facies of this gabbro we have an interesting and 

 important petrogenic problem. That problem can probably only 

 be solved by approaching it from the point of view of physical 

 chemistry. In the crystallization to which the orbules owe their 

 peculiar structure, in the spacing of the orbules, and in their 

 relation to the normal facies of the gabbro, we have various facts 

 the adequate discussion of which would seem to fall clearly 

 within the domain of the physical chemist. Yet it is difficult 

 for the physical chemist with his limited mineralogical and 

 petrographical experience to appreciate these facts, so that the 

 petrographer is constrained, rather than leave the problem wholly 

 unattached, to offer some feeble suggestions. 



We have in the orbicular gabbro magma differentiation pre- 

 sented in a two-fold aspect. The normal facies of the mountain 

 mass is a hornblende gabbro with some olivine and hypersthene. 

 The orbicular facies is a troctolite. Besides this primary differ- 

 entiation we have within the orbicular rock a secondary magmatic 

 differentiation. The ferro-magnesian uuisilicate olivine is, in 

 part, distinctly segregated from the aluminous lime polysilicate, 

 anorthite, the latter mineral crystallizing practically alone at the 

 core of many of the orbules and in some of the concentric shells. 

 The structure of th Q orbules as revealed by microscopic study 

 shows, moreover, that the magmatic differentiation has been 

 rhythmically recurrent 



If now we regard the magma as an intersolution of these two 

 silicates it w r ould seem possible to reduce one aspect of the prob- 

 lem at least to the terms of the "phase rule" of Willard Gibbs. 

 In such an intersolution the component which is in excess would, 

 with falling temperature, reach the saturation point first, and, by 

 crystallization, tend to maintain an equilibrium between the 

 interdissolved silicates which is definite for each particular tem- 

 perature of the cooling liquid mass. As the temperature falls, 

 however, a stage would be reached — the cryohydrate or eutectic 

 point — wdiere both silicates would crystallize together without 

 further diminution of temperature. If, while this simultaneous 



