199 



variation in colour and appearance, and contain both endogenous 

 and exogenous inclusions. We can distinguish quartziferous 

 forms and forms devoid of quartz, which again can be more 

 or less rich in orthoclase, plagioclase and ferromagnesian 

 minerals. 



First among these may be mentioned a variety, collected 

 as a block, of brick-red colour, with small, inconspicuous por- 

 phyritic crystals, among which felspar is predominant; yet, 

 especially in certain areas, a larger quantity of quartz stands 

 out. Microscopically we can see numerous little dihexahedra 

 of quartz, while the larger individuals are composed of felspar, 

 predominantly of plagioclase^ which is strongly weathered and 

 kaolinized. The ground-mass is microgranitic, yet it is quite 

 possible that what seems to be individuals consists really of 

 intimate intergrowths between quartz and feldspar. 



This variety has been analyzed by Miss Naima Sahl- 

 bom, B. A., and the following is the result of her analysis: 



SiO^ 75*14 per cent 



ГгОз 016 — 



Ah_0^ 12-50 — 



Fe^O^ 1-20 — 



FeO 0-87 — 



MnO traces 



CaO 0-83 — 



MgO 0-43 — 



K.^0 3-50 — 



l^a^O 3-00 — 



Loss at ignition 2*ô5 — 



100-18 per cent 

 Thus, in its chemical constitution the rock reveals nothing 

 especially striking. The considerable loss at ignition seems to 

 stand in connection with a radical decomposition of the felspars. 

 The proportion of soda is high, just as seems to be the case 

 with the more basic formations of the series. 



