Parciit-Roclx of the Diamond i)i South Africa. 103 



the rock in its unaltered condition was a coarsely holo- 

 crystalline mixture of chrome-diopside and garnet, with 

 a few small enclosures of olivine, — in other words, it was 

 a variety of eclogite and of igneous origin.^ 



2. A fragment (probably about one quarter) of a liattisli 

 oyoid boulder. — The two broken surfaces, which are 

 nearly at right angles, measure 5 and 5^ inches, roughly, 

 and it is about of inches high. The roclc yerj closely 

 resembles the one just described, except that mica occurs 

 rather oftener and in larger flakes : perhaps the garnets 

 (here also not quite regularly distributed) are slightly 

 more numerous. The outer surface is not quite so well 

 preserved, though enough remains to show that it also has 

 been smooth, and a few" thin veins of a white mineral 

 (calcite ?) traverse the rock. On this surface, near the 

 meeting of tlie two fractures, and exposed by the re- 

 moval of a little material (i. e., it might originally liave 

 been just hidden) is a diamond (octahedron), apparently 

 about 0.1 inch in diameter. On one side it rests against 

 a pyrope, the adjacent surface of which is incurved, the 

 two minerals being parted by the dull green-coloured 

 kelyphite rim of the latter, which is about 0.03 incli in 

 thickness. Thin sections of this boulder correspond 

 almost exactly with those from the other, the garnets 

 showing jjrecisely the same tints, though traces of a 

 cleavage (roughly parallel throughout) are perceptible on 

 close inspection, and are distinct under the microscope. 

 In garnet such a structure commonly indicates pressure, 

 and the general parallelism accords with this explanation, 

 but the other constituents show no signs of crushing. 

 The " kelyphite " rims to tlie garnets are perhaps slightly 

 broader, and the brown mica passes into a green (chlorite ?) 



1 I am, of course, aware that ef-Iogite, in the past, has been regaided by some 

 geologists as a nietamorphic rock. Apart from the fact that several rocks once 

 assigned to this class are now, with good reason, regarded as igneous, I have had 

 several opportunites of studying eclogite, and have no doubt as to its origin. Take 

 away the alkali from a magma with the clieraical composition of a diorite, and the 

 result would be garnets in place of felspar, i e., an eclogite. 



