208 The American Naturalist. [March, 



porphyrites and melaphyres. The author unfortunately classes as 

 diabase-porphy rites both glassy and holocrystalline rocks. The acid 

 rocks of the region include granites, granite-porphyries, porphyrites 

 and quartz-porphyries. The first two are characteristically granophyr- 

 ic. Their orthoclases are often enlarged by granophyre material 

 whose feldspar is fresh, while the nucleal feldspar is much altered. 

 The quartzes likewise, are enlarged by the addition of quartz around 

 them. There were two periods of crystallization in these rocks. In 

 the second period the phenocrysts were corroded and the groundmass 

 was produced. In addition to the quartz and orthoclase there are 

 present in these rocks also biotite, hornblende, plagioclase and a num- 

 ber of accessory and secondary components. The porphyries and 

 porphyrites contain the same constituents as the granites, from which 

 they are separated simply on account of differences in structure. The 

 phenocrysts are mainly orthoclose, plagioclase, microcline and quartz, 

 many of which are fractured in consequence of magma motions. The 

 groundmass in which these lie is of the usual components of porphyry 

 groundmasses, and in texture is microgram tic, granophyric, micropeg- 

 matitic and spherulitic. Many of the porphyries contain fragments of 

 their material surrounded by a matrix of the same composition in 

 which flowage lines are well exhibited. These rocks are evidently 

 volcanic breccias. The author divides the porphyritic rocks into por- 

 phyries and porphyrites, the latter containing plagioclase phenocrysts 

 and the former phenocrysts of quartz, orthoclase and microcline. 



Rocks from Eastern Africa.— The volcanic rocks of Shoa and 

 the neighborhood of the Gulf of Aden in Eastern Africa comprise a 

 number of varieties that have been carefully studied by Tenne. 3 The 

 main mass of the mountains of the region consists of biotite-muscovite 

 gneiss. This is cut by nepheline basanites, the freshest specimens of 

 which contain phenocrysts of olivine, augite and feldspar in a ground- 

 mass of plagioclase, augite, nepheline and often olivine. Trachytes, 

 phonolites and basalts occur in the Peninsula of Aden. The trachytes 

 include fragments of augite-andesite. Inland granophyres with pseu- 

 dospherulites in their groundmass, trachytes and feldspathic basalts 

 were met with. The granophyres are much altered. In the fine 

 grained product formed by the decomposition of the groundmass of one 

 occurrence quartz, feldspar, and a blue hornblende with the properties 

 of glaucophane can be detected. All the rocks are briefly described. 

 They present no peculiar features other than those indicated. 

 3 Zeits. d. deutsch. geol. Ges., XLV, p. 451. 



