398 SCIENCE PROGRESS 



now shown by Daly to be a sill, thereby greatly simplifying 

 the discussion of its origin and differentiation. Daly believes 

 in the origin of the red rock by the assimilation of sediments, 

 and Bayley's original views are much elaborated. The assimila- 

 tion is believed to have been masked by differentiation, which 

 had two phases, one controlled by gravity, the other by gas 

 tension. 



Bowen extends the hypothesis of gravity differentiation to 

 the origin of the anorthosites. These mono-mineralic rocks 

 show no signs of ever having been at the temperature necessary 

 for the crystallisation of plagioclase, and consequently have 

 probably never been molten as such, but have been formed 

 simply by the collection of plagioclase crystals from a complex 

 melt. The method of accumulation considered most likely is 

 that of the separation by gravity of the mafic constituents from 

 a gabbroid magma, while the plagioclase crystals remained 

 suspended. The consequences of this theory are shown to be 

 in accord with the evidence obtained by the field study of the 

 anorthosites of the Adirondack Mountains and of the Morin 

 area of Canada. 



The Lugar sill also provides striking evidence in favour of 

 gravity differentiation. Roughly speaking it consists of a 

 sheath of teschenite, enclosing a large mass of ultrabasic rock 

 which grades from theralite at the top, through picrite, to peri- 

 dotite. This gradation is characterised by an increasing pro- 

 portion of olivine downward. This is interpreted as being 

 due to the progressive sinking of the early-formed olivine 

 crystals, which accumulated toward the base, forming a peri- 

 dotic stratum. 



Smith describes a biotite-lamprophyre dyke intrusive into 

 Carboniferous shales. A remarkable feature is the inclusion 

 of xenoliths carrying corundum, staurolite, and magnetite. 



Lacroix proposes three new names for the long-suffering 

 petrographer. His ordanchite is the prevalent type of hauyn- 

 tephrite occurring on Mont Dore, Auvergne. Tahitite is a 

 variety of hauyn-tephrite much richer in hauyn and alkalies 

 than ordanchite. Among numerous other granular xenoliths 

 occurring enclosed in the Mont Dore lavas, Lacroix distinguishes 

 mareugite, which may be described as hauyn-bytownite- 

 gabbro. Its abundance as xenoliths is so great that it must 

 occur in situ beneath the volcano, 



