HORN EXPEDITION — PHTKOLOGY. 85 



ERUPTIVE ROCKS. 



Micro-pegmatite. No. 220. Platu I., Fig. 1, and Plate IV., Fig. 2a. 



This is a liolo-crystaliine granitic rock coiiipcscd of felspar, quartz, dark 

 ferruginous mineral patches, and wliat may be termed a groundmass of irregular 

 grains of micro-pegmatitically inter-crystallised quartz and felspar. There are only 

 a few scattered grains of quartz in addition to that inter-crystallised with the 

 felspar. There are, however, many crystals of felspar, some of them having 

 somewhat idiomorphic contours, while all are render'ed cloudy from the presence 

 of decomposition products. In addition there are several irregular dark patches 

 consisting principally of red oxide of iron, but also to a less extent of magnetite. 

 These patches occupy the sites of, and have resulted from the decomposition of, a 

 ferruginous mineral which has left insufficient evidence to enable its species to be 

 determined. The greater part of the rock is composed of inter-locking micro- 

 pegmatitic grains, the boundaries of which are not evident, until the rock is 

 examined "under crossed nicols." Tlu; drawiiig of the figure (Fig. 1) has been 

 made in ordinary light, and a few faint lines have been put in to indicate the 

 boundaries of the micro-pegmatitic grains as seen in polarised light. Several of 

 the gi-ains exhibit an irregularly divergent arrangement of quartz and felspar rays 

 l>ranching out from a piece of felspar occupying the centre of the grain. The 

 central piece of felspar is occasionally oljserved to be in optical continuity with the 

 felspar of the surrounding micro-pegmatitic mass. This tlivergent ai'rangement 

 corresponds somewhat to the granophyric structui'e of Rosenbusch. It has been 

 noticed that in one or two cases the central pieces of felspar cxhiljit simple 

 twinning, and that this arrangement is continuous throughout the external micro- 

 pegmatitic portion of the grain. 



In Plate I., Fig. 1, the felspar of the micro pegmatitic grains is shown by light 

 shading corresponding to its cloudy appearance under the U)icroscope. 



In Plate IV., Fig. 2(/, a small grain exhibiting the micro-pegmatitic structure 

 has been highly magnitiril and di'awn " under crossed nicols,'' the felspar being 

 extinguished and the triangular masses representing the (juartz. This specimen 

 formed part of a pebble from the Mount Olga conglomerate. 



Micro-pegmatite. No. 215. Plate I., Fig. 2, and Plate IV., Fig. 2/>. 

 Sp.gr. 2-71. 



This rock is ajiparcntly a variety of No. 22U, l)ut in this case tlu^ micro-peg- 

 matite is developed on a very fine scale, and is to be clearly maile out only under 



