NOTES ON HALLEY S COMET. 137 



of the great main tail. The Director of the Transvaal Observa- 

 tory has made the suggestion that at the near approach of the 

 comet to the earth a rupture occurred, the greater portion of the 

 tail remaining North of the Ecliptic whilst the nucleus, with a 

 shortened tail, passed on to the South. This hypothesis explains 

 the facts, but itself requires much explanation. The other theory 

 suggested, viz., that the tail lagged very much behind the line 

 joining the head of the comet and the sun does not appear to be in 

 accordance with observed facts. 



In connection with the rupture theory the great difficulty is to 

 account for the enormous forces which would be necessary to bring 

 to a stop particles moving with a velocity relative to the earth of 

 about forty miles a second. The theory of light pressure hardly 

 accounts for this ; our knowledge of the electrical properties of the 

 upper limits of the atmosphere is too meagre ; the key to the 

 mystery may lie within the comet itself ; at all events the visit of 

 Halley's Comet in 1910 seems to have left behind it more problems 

 than before. 



PORT SHEPSTONE MARBLE —The latest issue of the 

 Onarierly Journal of the Geological Society contains a paper 

 by Dr. F. H. Hatch and Mr. R. H. Rastall on dedolomitization in 

 the marble of Port Shepstone, Natal. The marble is well-known 

 in Natal, and is of a beautiful white appearance, and coarsely 

 crystalline in texture. Chemical analyses have shown it to be a 

 dolomite, containing 34 to 38 per cent, of Magnesium carbonate. 

 ^^'hether this dolomite was originally deposited as such during 

 the Swaziland period, or resulted from the alteration of a lime- 

 stone is not clear. Intrusive granite of a pre-Devonian age com- 

 pletely surrounds the dolomite, which is also traversed by broad 

 dykes. The authors, when visiting the marble quarries, were 

 struck by the appearance of a block of granite included in the 

 dolomite and surrounded by concentric dark brown, pale brown, 

 and green bands or zones. This interesting occurrence led to a 

 petrographical investigation. The granite block is either a soda- 

 granite or a soda-aplite, and is surrounded in the first place by a 

 dark coloured mica-olivine band, succeeded by a light mica-for- 

 sterite zone, and that again by an aggregation of white carbonate 

 and greenish-yellow serpentine. Other "inclusions were also no- 

 ticed in the dolomite, and of these a felspar-scapolite-diopside 

 rock and a spinel-forsterite rock are described in detail. The 

 authors express the view that the included blocks of rock were 

 deposited in the dolomite at the time of its formation, and that 

 dedolomitizat'on of the original rock occurred where silica from 

 extraneous sources was available, silicates being formed in such 

 cases. The included block of granite is unique amongst South 

 African rocks, differing wholly from the grey granite of the cir- 

 cumjacent district. The dolo'mite and its included granite, the 

 authors hold, were subjected at the time of marmorization to' very 

 high temperature and great pressure in the presence of water 

 vapour. ,. 



