224 



PETROLOGY OF ROCK COLLECTIONS 



A comparison of the analyses of the granites and lamprophyre is worthy of note ; 

 these are tabulated below : 



THE DIORITES 



As suggested under the last section, there is reason to believe some of these at least to 

 be genetically closely connected with the earlier granites — probably lamprophyric 

 differentiation products. As nothing is yet known of the occurrences, in situ, of the 

 specimens hereunder described this point cannot now be cleared up. The rocks are of 

 such special interest that detailed descriptions are warranted. 



Sphene-Biotite-Hornblende Diorite ; near Mount Larsen 



This was found as an erratic boulder from a moraine on the ice-fringe about 20 miles 

 south-east of Mount Larsen (Fig. 5, Plate IV). In the hand-specimen this rock is 

 even-grained and speckled-grey in colour. The obvious minerals are plagioclase, 

 hornblende, mica, and beeswax-yellow sphene. 



Microscopic Characters. Hypidiomorphic-granular with grain size, in the section, 

 averaging 0'7 mm. (Fig. 1, Plate IV). A considerable amount of granular quartz is 

 present, often exhibiting shadowy extinction ; it is certainly one of the latest crystal- 

 lised products. It is doubtful if any orthoclase is present, none could be recognised 

 n the two slides. 



Plagioclase is present in abundance, ranging from acid labradorite to basic oligoclase. 

 The more basic individuals are early idiomorphic crystallisations. Zoning is almost 

 universal. Biotite is abundant ; pleochroism — light yellow, bronze, to deep yellowish- 

 brown. Hornblende is plentiful ; extinction angle about 13 degrees ; pleochroism, 

 light yellow, yellowish-green, to dark green. The hornblende encloses biotite. Sphene, 

 light yellow by transmitted light, dull grey by reflected light ; grains often 2 mm. in 

 length ; in some sections two sets of cleavage are clearly shown ; crystallisation of the 

 sphene appears to have taken place after much of the felspar. Magnetite, apparently 

 titaniferous, appears in small not abundant grains usually embedded in the biotite. 

 Pyrites in scattered grains is more frequent than the magnetite. Apatite in very small 

 laths is rather abundant, usually embedded in the felspars. Occasional small grey 

 rods of zircon highly refracting and with straight extinction. 



