206 



phibole mineral has originally entered into the composition 

 cannot be determined. 



Finally we come to what is perhaps the most interesting 

 of all these types ^), and which was the object of detailed ex- 

 amination, though even now its nature cannot be quite deter- 

 mined. Macroscopically it is a grey, compact, almost hornstone- 

 like rock, with numerous porphyritic crystalline agglomerations 

 of a hght, bronze-coloured mica; other phenocrysts, whether 

 lighter or darker, only appear on the polished surface. 



Under the microscope we can distinguish now biotite of 

 the ordinary appearance, often in the form of hexagonal scales, 

 now a yellowish green, not especially pleocroitic, fresh horn- 

 blende with maximal extinction of 16°, then furthermore one 

 or two light, much transformed minerals, the nature of which 

 is more difficult to determine. Some of them are individuals 

 with idiomorphic outlines of approximately hexagonal form and 

 consisting partly of a carbonate mineral, partly of an almost 

 isotropic, serpentinic mass. I do not think it possible that 

 here a felspar was originally present. However, it looks as if 

 this mineral were connected by transitions with others which 

 by their weak, flamy, varying double refraction indicate that 

 they consist of sub-microscopic intergrowths; as a rule they 

 do not contain carbonate, but they contain other inclusions or 

 decomposition products, which in part make them almost iso- 

 tropic. The shape is often rectangular or rectangular with 

 blunt truncations on two sides. The double refraction recalls, 

 in the purest sections, felspar, and as felspar we should class 



'^) While on a visit to Heidelberg I had an opportunity of showing a few 

 of the specimens here described to Professor H. Rosenbusch, and 

 in the main he confirmed the opinions I had myself arrived at. I was 

 especially interested in hearing from a scientist of such ripe experience 

 an opinion about these particular rocks; he compared them to certain 

 trachy-dolerites and tephrites, and especially to nepheline-tephrites from 

 the area of the large East-African depressions. I seize this opportunity 

 of expressing my sincere thanks to Professor Rosenbusch for so 

 kindly placing his time at my disposal. 



