Parent-Rock of tlie Diamond in South Africa. Ill 



Other specimens of the sedimentary rock in the im- 

 mediate neighbourhood of tlie blue have been forwarded 

 to me by Mr. Trubenbach, one, from the adit on the 

 southern side of the section mentioned above, is a grey 

 mudstone, containing a flattish rectangular pebble, of a 

 dark green compact rock. Two others are from No. 2 

 mine, or about 700 yards to the south-west. One, struck 

 in the shaft at a depth of 200 feet, is a conglomerate, 

 composed of well-rounded rock fragments, with some 

 scattered grains of quartz. Each of the foiaiier is bordered 

 by a zone of the crystalline carbonate (impure calcite), 

 and the interstices are filled, sometimes by a clearer 

 variety of the same, but more often by some minutely 

 granular secondary product. Of the rock fragments, one 

 is a sub-crystalline dolomitic limestone ; two, perhaps, are 

 chalcedony ; the remainder are igneous ; the majority 

 being varieties of the diabase, sometimes rather decom- 

 posed ; the rest trachytes, mainly andesites. Their general 

 aspect and the not unfrequent presence of vesicles (now 

 filled with viridite) suggests that they have been furnished 

 by lava-flows. Another specimen, obtained in the same 

 working at a depth of 400 feet, is a rather felspathic dia- 

 base, not unlike one of the varieties in the conglomerate. 

 It is a good deal decomposed, is not improbably from a 

 lava-flow, but does not call for a minute description. 



Conclusion. 



Thus the diamond has been traced up to an igneous 

 rock. The " blue ground " is not the birthplace either of 

 it or of the garnets, pyroxenes, olivine, and other minerals, 

 more or less fragmental, which it incorporates. The 

 diamond is a constituent of the eclogite, just as much as a 

 zircon may be a constituent of a granite or a syenite. Its 

 regular form suggests not only that it was the first mineral 

 to crystallise in the magma, but also a further possibility. 

 Though the occurence of diamonds in rocks with a hioh 



