452 ORVILLE A. DERBY 
In a recent article Irving" has discussed the formation of com plete 
crystals in rock masses. ‘Those that are of primary origin in igneous 
rocks were, naturally, formed in a magma that was mobile through 
fusion, while those of secondary origin are attributed to replace- 
ment. In sedimentary or other rocks already consolidated, space 
for such crystals could, according to Irving, only be gained through 
displacement of granules of the enclosing rock by virtue of the force 
of crystalline growth, or through the removal by solution of such 
molecules of the rock as occupied the space to be taken up by the 
crystal in process of formation. 
The minerals mentioned by Irving as having had their lodging- 
place prepared by solution represent sulphides (pyrite, galena), 
carbonates (siderite), fluorides (fluorite), and boron-bearing 
silicates (tourmaline), from which it may be inferred that the 
requisite solvent power is an attribute of the so-called mineralizing 
agents: sulphur, carbon in some one or more of its gaseous forms, 
fluorine, and boron. 
No precise statement regarding the character of the molds left 
by the dislodgement of diamond crystals from their parent-rock, 
kimberlite, is at hand, but judging from the current hypotheses 
(formed in place or floated up from some pre-existent rock), these 
should be as sharp-cut and perfect as those of the above-mentioned 
replacement minerals. Such perfect molding occurs with minerals 
formed in mobile (molten) media, and in the case of the diamond 
no other hypothesis seems to have ever been considered, but with 
our present scanty knowledge of the obscure subject of the genesis 
of that mineral the hypothesis that it may be due to replacement 
cannot be lightly put aside. 
The studies of Beck? and Bonney? on the so-called eclogite 
nodules from the Newlands mine offer some support to this last 
hypothesis. A number of these were examined by both authors, 
but apparently only a single diamond-bearing one was ever found, 
™“ Replacement Ore Bodies and the Criteria for Their Recognition,” Economic 
Geology, October-November, roit. 
“Untersuchungen iiber einige siidafrikanische Diamantenlagerstatten,’ Zeit. 
d. deutsch. geol. Gesell., 1907, pp. 290 f. 
3 ‘The Parent-Rock of the Diamond in South Africa,’ Proc. Roy. Soc., LXV 
(1899), Pp. 220. 
