/row th>- X<'n-iii,iilx Diiiiiiom) Mine.-;, Gru^'nlo n>l JJVs/. 479 



Professor Kosenbusch however mentions the occurrence in .some syenites 

 (including those with elseolite), in diorite (very rare), and in a hyper- 

 sthene gabbro (or norite) from P^kersund, on the west coast of Norway, 

 and St. Paul's Island, Labrador.* Thus we may be content to call this 

 rock a pegmatitic hornblendic Gabbro. 



(/<.) This perhaps represents a pebble rather than a boulder, for it is 

 a fragment only about L! x 1-J x 1 inches, adhering to a piece of " blue 

 ground," the surface in contact with the latter being well rounded. 

 Macroscopic-ally it appears to be a medium-grained diorite ; the micro- 

 scope shows a holocrystalline granular structure; the plagioclastic 

 felspar is in fair preservation, and, perhaps, is labradorite ; the horn- 

 blende is rather strongly pleochroic, ranging from pale brownish-green 

 to deep brown. The mineral, however, is not original, but an alteration 

 product from a pale green augite (omphacite ?). Grains of iron oxide 

 are also present. Slight decomposition has taken place in a narrow zone 

 from the surface inwards. 



(?'.) The last specimen is a lump of irregular shape. Presumably it 

 is from the blue ground, but there is nothing to prove this. In a 

 compact dark brown to slightly purple ground-mass, a number of 

 irregularly-formed greenish-grey patches are scattered so as to suggest 

 flow brecciation. These, when examined under the microscope, are a 

 very light greyish-brown in colour, exhibiting flow structure, minute 

 devitrification, and some decomposition. The matrix is darker, 

 sprinkled with opacite and ferrite, minutely devitrified, showing an 

 irregular wavy structure, and occasionally ill-defined crystallites of 

 plagioclase felspar. The rock, now a felsite or porphyrite, was prob- 

 ably once either a sanidine trachyte or more probably an andesite, 

 with flow brecciation. This specimen possibly may not represent a 

 boulder, but a dyke or flow associated with the " blue ground." 



(2.) Diamantiferow Matrix. 



Specimens of the "blue ground" in which the boulders occurred 

 were also sent. As they came from another part of the mine, and the 

 best preserved exhibited one or two slight differences, I have had a 

 few slices prepared. To the unaided eye the matrix is more of a 

 purple-brown colour, slightly more compact and hard, but more brittle ; 

 the fragments of magnesian minerals, however, seeming more com- 

 pletely serpentinised. A few small, rather crumbling, rock fragments, 



* ' Elements cler Gesteinslehre ' (1898), p. 221. A case where the structure is 

 more like that of the true graphic granite, from the dolerite of Pouk Hill, is 

 described by Mr, Allport, 'Quart. Journ. Geol. Soc.,' vol. 30, p. 549, and figured by 

 Mr. Teall, ' British Petrography,' PI. XXIII, fig. 2. An instance of micrographic 

 intergrowth of quartz and calcite is described by Mr. Cooniura-Swamy in the afore- 

 named journal, vol. 56, pp. P05, 606. 



