228 



Prof. T. G. Bomiey. 



and obtain on a clinopinacoid an extinction of 35° with a prism edge. 

 It is in fact identical with the pyroxene described by Professor Lewis* 

 as chrome-diopside. In it (though rare) are small rounded enclosures 

 of a greenish mineral aggregate much blackened with opacite. I 

 regard them as alteration products of a ferriferous olivine. This 

 diopside, at the exterior and along cracks, is often converted into 

 a minutely granular to fibrous mineral, which gives a " dusty " aspect 

 to that part of the crystal, when viewed with transmitted light, and 

 a whitish-green one with reflected light. This often terminates in a 

 minutely acicular fringe, piercing the original diopside. Its grains 

 occasionally are a little larger, showing a cleavage, dull green in colour, 

 fairly pleochroic, and having the extinction of hornblende. A process 

 of secondary change, as in uralite, is no doubt indicated. Xow and 

 then a tiny film of brown mica occurs in this part or even in a crack 

 in the diopside. 



It is this alteration product which gives the mottled aspect men- 

 tioned above as visible to the unaided eye, so this is not indicative of 

 a third important constituent in the original rock. In one of the 

 slices the mica just named attains a larger size (about 0*03 inch across), 

 has a fairly idiomorphic (hexagonal prism) outline, and is not restricted 

 to the margin of the garnet. In this case it is generally associated 

 with calcite,f which it tends to surround, and that in one place encloses 

 a radiating acicular mineral (? a zeolite) ; in another the calcite, or some 

 other carbonate, is mixed with a serpentinous material. Distinct 

 granules of iron oxide are practically absent from the slices, though 

 here and there it may be indicated by some opacite. I have not found 

 spinel, or rutile, or zircon, or pseudobrookite. In fact, putting aside the 

 diamonds, 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 flattish ovoid 

 boulder. — The two broken surfaces, which are nearly at right angles, 

 measure 5 and 5 J inches, roughly, and it is about 3J inches high. The 

 rock very 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 



* Loc. cit., p. 21. 



f From the facts I think it probably of secondary origin. It reminds me some- 

 times of the brown mica produced by contact metamorphism. 



X I am, of course, aware that eclogite, in the past, has been regarded by some 

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

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

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

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

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



