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462 W. D. Lang — Key to some Cretaceous Polyzoa, 



(precious) 'catseye' and the form known as 'alexandrite,' wliicli is 

 green by day- and red by candle-light, also occur. This mineral 

 •would seem to be usually a product of contact-metamorphism ; at 

 the same time, although it is not strictly analogous in a chemical 

 sense, its similarity of composition and isomorphism with olivine 

 are to be noted. As a gem the ordinary variety is actuallj"^ called 

 ' chrysolite,' one of the names properly applied to olivine, Kyanite 

 and sapphire are unequivocal contact minerals, so are rutile and 

 tourmaline, whose occurrence may also be noted. The presence of 

 the first-named is interesting owing to its softness, which makes its 

 survival rather remarkable. Of the sapphires both the blue and 

 colourless varieties occur, while true rubies and Oriental amethysts 

 are also found, though they are distinctly rare even for such scarce 

 stones. Another stone whose occurrence may be noted is the so- 

 called ' Somabula blue.' This is harder and heavier than common 

 beryl, and is possibly a variety of topaz ; when cut it is one of the 

 most beautiful gems imaginable. 



With regard to the origin of the gems, the mere richness of the 

 deposit is sufficient to indicate a near source for the diamond, 

 although many of the constituents of the gravel itself have 

 obviously travelled far. Despite theories to the contrary, it seems 

 certain from the evidence obtained in New South Wales as well as 

 in this country that what is commonly called ' blue ground ' is in 

 all cases the original source of the diamond, and the great 

 abundance of enstatite, as well as the presence of garnets, points 

 to the same origin in the case of the Somabula field. I am aware 

 that Professor Gregory after a hurried inspection of the gi'ound has 

 pronounced the opinion that the diamond comes from pegmatite 

 veins, but such an idea is so completely at variance with the local 

 conditions and with all that we know of diamond occurrences that 

 it scarcely merits discussion. I have little doubt that it will not be 

 long before the pipe which produced the diamonds is discovered, 

 and that it will present, apart from slight local peculiarities, all the 

 usual features of the South African mines already known. 



V. — A Key to the Published Figures of the Cretaceous 



Forms of the Polyzoan genus Entalophosa. 



By W. D. Lang, M.A., F.Z.S., F.G.S., of the British Museum (Natural History). 



IN view of the vigour with which the investigation of the English 

 Chalk is at present being carried out by amateurs, resulting in 

 the accumulation of large numbers of fossil specimens which give 

 rise to difficulties of nomenclature, the publication of a key to aid 

 collectors in determining the specific position of members of one of 

 the largest Cretaceous genera of the phylum Polyzoa does not seem 

 inappropriate. 



The most inclusive work on Cretaceous Polyzoa is the British 

 Museum Catalogue. While this gives descriptions of such species 

 as the British Museum possesses, with figures of many, and 

 references to all the literature on the group, it does not present any 



