72 DE. J". BALL ON THE [Feb. I903, 



The syenite-porphyry dykes show less crushing than any of the 

 other igneous rocks near them, and are probably the youngest 

 igneous rocks in the district. 



The Hornblende-Schist. 



This rock forms a considerable patch on the left bank just above the 

 Semna temple. The relations of this rock to the gneiss are not very 

 clear, but it appears to form a band in the latter. In correspondence 

 with its more basic composition, it is much more highly weathered 

 than the gneiss, and forms a less prominent feature in the landscape. 

 Its specific gravity is 2 # 96. Like the gneiss, this rock also proves on 

 microscopic examination to be a deformed igneous rock, being in fact 

 a somewhat crushed fine-grained diorite. Hornblende, the most 

 abundant constituent, occurs in highly pleochroic, allotriomorphic 

 crystals (pale yellow to deep blue-green, extinction-angle c — c = 18°). 

 The felspars, which with the hornblende make up the bulk of the 

 rock, appear to lie between andesine and labradorite, sections per- 

 pendicular to the brachypinacoid giving extinctions up to 25°. 

 Very small amounts of quartz, and a brown biotite, the latter inter- 

 grown with the hornblende, are present; and there are a few 

 idiomorphic crystals of apatite, as well as grains of haematite, epidote, 

 and perhaps sphene. The microscopic slide here shows far less 

 evidence of crushing than in the case of the gneiss : the schistose 

 appearance is only well seen in the rock-mass. Hence it would 

 appear likely that the rock represents an intrusion into the gneiss 

 subsequent to the main crushing operation. 



The Augitite. 



One of the specimens taken from what was thought in the field to 

 be a more basic development of the ' hornblende-schist ' above men- 

 tioned, proves on microscopic examination to be an augitite of an 

 interesting character. The rock, which is verjr fine-grained and 

 dense (specific gravity = 3*24), is of a blackish-green in the mass. 

 The thin section shows it to be holocrystalline, and to consist entirely 

 of augite, hornblende, and sphene, with a small amount of interstitial 

 quartz. The augite, which forms about five- sixths of the slide, and 

 of which there is no trace of more than a single generation, is almost 

 colourless, having only a faint brown tinge ; it forms a mosaic of 

 more or less rounded granules, with frequent irregular cracks and 

 well-marked cleavages (see PI. IV, fig. 1). Prismatic sections give 

 extinction-angles up to 42°. Scattered through the augitic mosaic 

 are irregular straggling crystals of green hornblende, and large 

 granules of sphene ; these last two occur in about equal abundance. 

 The sphene, though mostly in rounded grains, is sometimes seen in 

 approximately idiomorphic forms, being the only constituent of the 

 rock to appear thus : the grains are about as large as those of the 

 other minerals present, and show strong pleochroism (almost 

 colourless to a rather deep pink-brown). The little quartz present is 

 in the form of small interstitial grains, and is probably of secondary 



