24 A. J. Jukes-Browne — Hed Chalk in Suffolk. 



fragments of carbonaceous shale, while the non-diamantiferous variety- 

 is apparently free from all inclusions, and is a typical volcanic rock. 



Both are dark, heavy, basic rocks, composed essentially of olivine, 

 and belong to the group of peridotites. Both are of similar structure 

 and composition, differing only in the presence of inclusions. The 

 rock consists mainly of olivine crystals lying porphyritically in a 

 serpentinic ground-mass. 



The olivine is remarkably fresh, and occurs in crystals which are 

 generally rounded by subsequent corrosion. The principal accessory 

 minerals are biotite and enstatite. The biotite is in crystals, often more 

 or less rounded, and sometimes surrounded by a thin black rim, due to 

 corrosion. Similar black rims surround biotite in many basalts. The 

 biotite crystals are usually twinned according to the base. The 

 enstatite is clear and non-pleochroic. Garnet and ilraenite also occur, 

 the former sometimes surrounded by biotite, and the latter often partly 

 altered to leucoxene. All these minerals lie in the serpentinic base, 

 originally olivine. This rock appears to differ from any heretofore 

 known, and may be described as a saxouite porphyry in which the base 

 is not holocrystalline. 



The diamond-bearing portions often contain so many inclusions of 

 shale as to resemble a breccia, and thus the lava passes by degrees into 

 tuff' or volcanic ash, which is also rich in diamonds, and is more readily 

 decomposable than the denser lava. 



It seems evident that the diam.ond-bearing pipes are true volcanic 

 necks, composed of a very basic lava associated with a volcanic breccia 

 and with tuff, and that the diamonds are secondary minerals produced 

 by the reaction of this lava, with heat and pressure, on the carbona- 

 ceous shales in contact with and enveloped by it. 



The researches of Zirkel, Bonney, Judd, and others, have brought to 

 light many eruptive peridotites, and Daubree has produced artificially 

 one variety (Iherzolite) by dry fusion, but this appears to be the first 

 clear case of a peridotite volcano with peridotite ash. 



Perhaps an analogous case is in Elliott County, Kentucky, where 

 Mr, J". S. Diller has recently described an eruptive peridotite which 

 contains the same accessory minerals as the peridotite of Kimberley, 

 and also penetrates and incloses fragments of carboniferous shale, thus 

 suggesting interesting possibilities.-' 



V. — I^OTE ON A Bed of Red Chalk in the Loweb, Chalk oe StrEFOLK. 



(A paper read before the British Association, Birmingham.) 



By A. J. Jukes-Browne, B.A., F.G.S. 



THE section exposing this stratum was discovered during an excursion 

 made last June by Mr. W. Hill, F.G.S., and the author. It occurs 

 in a quarry near West Eow Ferry, about two miles west of Mildenhall ; 



^ Since this paper was read, additional evidence that diamonds originate by the 

 action of peridotites on carbonaceous rocks has been collected from many localities. 

 The most instructive examples are from New South "Wales, where the diamond 

 gravels lie near the contact of basalts or serpentine with Carboniferous rocks, the 

 serpentine here being an altered olivine enstatite rock. At the Bingera diamond-field 

 a mass of eruptive serpentine is almost surrounded by Carboniferous rocks containing 

 coal-seams. In Western America, also, the diamantiferous gravels are near higher 

 ground, where serpentines and carbonaceous rocks occur together. Possibly a clue 

 may be thus afforded for more systematic search. 



