52 Rocks of Noyang. 



is the most frequent. It often replaces a large part of the 

 crystal. Another alteration product is kaolin. Minute flakes 

 of some chlorite mineral are also very frequent in these 

 felspars. The optical measurements which I was able to 

 effect in a few of the least altered felspars gave for the zone 



OP— co P co from 3° 30' to 10° 15'. The crystals are either 

 compounded according to the Albite law, or more rarely 

 simple. The long and narrow forms of these crystals show 

 that the elongation has taken place in the direction of the 



edge oo P cc — co P co 



(2.) Among and between the clustered felspar prisms are 

 numerous crystals showing the usual external angles of 

 amphibole, but having only the faintest traces of prismatic 

 cleavage. In sections which might be considered near the 

 clinopinacoid I obtained measurements of the inclination of 

 the plane of vibration as high as 18° 30'. This mineral is 

 polychroic, the colour of the three rays being, however, in 

 light shades — 



c — green ; b — light green ; a — pale yellow. 



Associated with the amphibole are numerous rectangular 

 crystals resembling magnetite, but, as some of them are 

 surrounded or partly composed of a somewhat opaque grey 

 material, they may rather be titanic iron, and the grey 

 material leucoxene. The large percentage of Ti0 2 shown 

 in the annexed analysis agrees with this view. 



Some yellow crystalline grains with rugged surfaces sug- 

 gest sphene. 



Besides these constituents there are, in spaces between the 

 other crystalline minerals, radial masses of some chlorite 

 mineral, which is, however, clearly distinguishable from the 

 other chlorites produced by the alteration of amphibole, and 

 I suspect that it may represent portions of basis. 



The sequence of formation of minerals in this rock is not 

 quite clear, but the probability is that the order has been as 

 follows: — (1.) Titanic iron; (2.) amphibole and plagioclase, 

 these two not differing perceptibly in period of formation. 



I subjoin a quantitative analysis of this rock, but, in the 

 absence of more precise knowledge of the composition of 

 the numerous alteration products, I have not found it prac- 

 ticable to calculate satisfactorily the mineral percentages. 

 All that I can say with safety is that the felspar is probably 

 a more basic one than is found in those rocks which I have 

 already described. 



