558 MR G. W. TYRRELL. 



mineral in place of ilmenite and anatase. It is very rich in apatite, and contains 

 abundant calcite as the result of the alteration of the auoite and melilite. In the 

 absence of olivine and garnet, the Ochilesa rock has closer affinities with the biotite- 

 monchiquite or ouachitite of Arkansas than to the type alnoite. According to 

 Rosenbusch * the ouachitite of Hot Springs, Arkansas, contains a small quantity of 

 melilite, and is free from garnet. It is included by him in the alnoite group. 



VII. Basic Intrusions. 



A group of basic rocks including dolerites and basalts is widely spread over the 

 area traversed by Professor Gregory. Their geological relations suggest that they 

 are the youngest set of igneous rocks in the region. 



A rock from the railway W. of Lepi (127) is a coarse-grained ophitic olivine- 

 dolerite, consisting of a plexus of broad laths of labradorite (AbiAni) in ophitic re- 

 lations with irregular plates of brown augite. There is also some anhedral olivine, 

 magnetite-blackened, and partly altered to colourless serpentine ; and titaniferous 

 iron-ore. The rock is mesocratic or mafelsic, i.e. the felspars are roughly equal in 

 amount to the ferro-magnesian minerals. 



A similar rock forms a dyke W. of Cruz's, Quingenge (236). It is, however, 

 finer in grain, and the iron-ore is more abundant, than in the Lepi rock. The augite 

 has a marked purplish tone. 



Rocks from near Cubal (300c) and Portella Solo (118) are very basic basalts 

 approaching augitite. The Cubal rock contains numerous large phenocrysts of brown 

 augite in a fine-grained groundmass consisting of augite and plagioclase, of which the 

 former is by far the more abundant. The texture is obscurely ophitic, and the augite 

 phenocrysts enclose very small laths of plagioclase. The only remaining consti- 

 tuents are ilmenite passing over to leucoxene, and small green patches which may 

 represent a little original olivine. The rock from Portella Solo differs from the above 

 only in its non-porphyritic character and the slightly greater proportion of the 

 felspar. It occurs at the margin of a granite mass, and appears to have been re- 

 crystallised as a basalt-hornfels, with little or no new mineral formation. Both 

 these rocks are practically devoid of olivine, and in the abundance of augite approach 

 the augitite end-member of the basalt family. 



EXPLANATION OF PLATE. 



Fig. 1. Charnockite, Ochilesa. Ordinary light, x 15. A granulitic aggregate of clear quartz, cloudy 

 felspars, with biotite (centre of field) and pyroxenes (below biotite). The latter is mostly hypersthene. 



Fig. 2. Hornblende - hyperite, Ochilesa. Ordinary light, x 15. A granulitic aggregate of felspar 

 (plagioclase) with a little quartz, abundant hypersthene, and hornblende. . The hypersthene is easily dis- 

 tinguishable by its comparatively high relief. 



* Rosenbusch, Gesteinlehre, 1910, p. 305. 



