50 JOURNAL OF GEOLOGY— SUPPLEMENT 



It has been pointed out in the discussion of crystalHzation that 

 the olivine-rich portion (peridotite) would be completely crystalline 

 while the remaining portion was still liquid and that the same is 

 true of the gabbro portion with respect to the siliceous differentiate. 

 If gravitative differentiation took place under perfectly quiet 

 conditions, the relations between the types would be transitional, 

 but any disturbance of the mass during consolidation might cause 

 the gabbro to have an intrusive relation to the peridotite, owing to 

 the injection of dykes into cracks, and the granite to have a similar 

 relation to the gabbro. In the Duluth laccolith the granitic phase 

 has this intrusive relation. 



Harker describes the intrusive igneous complex of the Cuillin 

 and Red Hills as consisting principally of three laccoliths of perido- 

 tite, gabbro, and granite respectively."" In view of what has been 

 ascertained concerning the course of crystallization in artificial 

 melts which have a direct bearing on the question of crystalliza- 

 tion of basaltic magma, the writer is emboldened to offer the follow- 

 ing alternative suggestion. The igneous mass referred to may be, 

 for the most part, one laccolith with a general eastward dip, formed 

 as the result of a single principal act of intrusion of magma of the 

 same composition as that extruded in colossal quantity during the 

 fissure-eruptions. Rapid cooling of this magma in the flows gave 

 basalts rather unusually rich in olivine. Slow cooling of the same 

 magma in the laccolith, accompanied by sinking of crystals, would 

 give, as we may deduce from experimentally ascertained facts, 

 spinel-hearing peridotite^ toward the western margin or base, gabbro 

 at intermediate levels, and granite toward the eastern margin or 

 top, the granite itself becoming more acid eastward, as Harker has 

 observed. The intrusive relation of the gabbro against peridotite 

 and of granite against gabbro would then be referred to move- 

 ments during consolidation, of which there is abundant evidence. 



' "The Tertiary Igneous Rocks of Skye," Mem. Geol. Survey Untied Kingdom, 

 1904. 



^ It is possible that even the common types of basaltic magma, not especially rich 

 in olivine, wiU give spinel as an early precipitate if very slowly cooled, for in the reverse 

 process — slow heating of the Palisade diabase of New Jersey — Merwin has witnessed the 

 formation of a spinel-Uke mineral. Cf. Sosman and Merwin, "Data on the Intrusion 

 Temperature of the Palisade Diabase," Jour. Wash. Acad. Sci., Ill (1913), 392. 



