54 . ^o[- XLIIL, Art. 5.— T. Katj: 



keen obsei'ver. On close examination under the microscope, these 

 rocks are always found to be intensely sideritized. 



The reddish brown colour of the rock is due to the presence 

 of abundantly and irregularly scattered flecks and innumerable 

 microscopic netted veinlets of iron hydroxide. The iron hydroxide 

 is yellowish brown in colour and shows indistinct a^^re^ate' 

 polarization- colours due to aggregation of microscopic grains. This 

 is cryptocrystalline limonite.^) That the limonite is derived from 

 siderite by oxidation and hydration is clearly indicated by the 

 following facts : — (1) Sometimes, in the interior of the flecks and 

 veinlets of limonite a core of unaltered siderite is preserved ; (2) 

 the brownish veinlet often passes gradually into a ve^n consisting 

 of strongly pleochroic carbonate (siderite) ; (3) in places, the country 

 rock is heavily impregnated with siderite flecks, and is penetrated 

 by netted veinlets of the same mineral, along the borders of which 

 a change to brown hydroxide of iron is frequently observed (PL 

 III., Fig. 1). 



Small amounts of chlorite in irregular flakes have been formed 

 in the stage of sideritization, since they are closely associated with 

 siderite flecks in the sideritized rock and in the veinlets of siderite. 

 On the other hand, the siderite flecks overlap the quartz grains in 

 the silicified slate, and the siderite veinlets cut the same rock. 

 The chloritized slate, too, is sometimes heavily impregnated with 

 siderite and is cut irregularly by netted veinlets of the same 

 mineral. Consequently, it is beyond all doubt that the sideritization 

 has taken place subsequent to the intense chloritization and silicifi- 

 cation of the country rock. 



In the country rock adjacent to the veins, no alterations charac- 



1) A. F. Rogers, " A Eeview of the Amorphous Minerals." Journ. Gtol, Vol. XXV., 1917, 

 pp. 528-529. 



