﻿part 2] 



EICBITE-TESCHENITE SILL OF LUGAR. 



125 



described in the next section. This, in its turn, was intruded at a 

 later stage, probably while still partly liquid, by a small mass of 

 lugarite, which spread out as a thin sheet at a horizon about a 

 third of the depth of the ultrabasic stratum from its upper surface. 



If the emplacement of the Lugar sill took place in this way, it 

 is difficult to understand why there are no xenocrysts or xenoliths, 

 intrusive veins, or signs of disturbance, along the main interior 

 contacts. The interposition of the ultrabasic magma must have 

 taken place very quietly and gradually, welding itself intimately 

 to the teschenite without wedging off fragments from the contacts. 

 That tbe teschenite was fractured and comparatively cool before 

 the intrusion of the picrite is shown by the presence of the black 

 veins of basaltic teschenite, which represent teschenitic magma 

 chilled by intrusion into cold, or comparatively cold, rocks. 



(10) Sinking of Crystals in the Central 

 Ultrabasic Stratum. 



Dr. It. A. Daly and others have shown that the earlier and 

 generally heavier minerals must tend to sink in the ordinary silicate 

 magma. Actual cases of phenocrysts that have sunk (or risen) 

 in lavas have been described by authorities of no less weight than 

 Scrope, Darwin, King, E. S. Dana, and Iddings. 1 In the great 

 quartz-dolerite sill of the Palisades (New Jersey) J. V. Lewis has 

 shown that a concentration of olivine has taken place near the 

 lower contact, giving rise to a stratum of olivine-dolerite. 3 This is 

 interpreted as being due to the sinking of early-formed olivine 

 crystals. Dr. Bowen has shown that olivine- and pyroxene- 

 crystals collect towards the bases of crucibles containing a suitable 

 silica melt, but that olivine sinks more readily. 3 That the process 

 cannot be more often demonstrated in natural occurrences is 

 probably due to the fact that in most small magmas the onset of 

 an inhibitive viscosity, or of crystallization, is too rapid for the 

 gravitational action to take place ; and in large magmas the final 

 products of the process are not often exposed by erosion. The 

 sinking of olivine-crystals is believed to have taken place in the 

 central ultrabasic stratum of the Lugar sill, where a downward 

 sticcession from theralite, through picrite, to peridotite, may be 

 demonstrated, a succession characterized by a gradually increasing 

 proportion of olivine. 



Dr. Daly has calculated that a holocrystalline ' basalt ' of sp. gr. 

 3 "10 at ordinary temperatures would have a specific gravity of 

 only 2-83 when molten at 1200° C. The figure 3'10 may be taken 

 as a fair average for the specific gravity of picrite, and we may 

 therefore conclude that picrite molten at 1200° C. woidd have a 



1 See R. A. Daly, ' Origin of Augite-Andesite ' Journ. Geol. Chicago, vol. xvi 

 (1908) p. 411. 



2 Ann. Rep. Geol. Surv. New Jersey 1907, pp. 125, 129-33. 



3 Amer. Journ. Sci. ser. 4, vol. xxxix (1915) pp. 175-91. 



Q. J. G. S. No. 286. l 



