30 Rocks of Noyang. 



high, in one case, as 22°, but in the majority of cases between 

 12° and 20°. 



Besides these, which may be considered with some cer- 

 tainty as hornblende, I found occasional instances of a second 

 amphibole of a somewhat different character. The most 

 marked feature, at first sight, was that its crystalline masses 

 included numerous fragments or imperfect crystals of 

 twinned felspars of small size. The outlines of this 

 amphibole are even more irregular than those of the just 

 described hornblende. The polychroism is much less, and 

 the angles formed by the plane of vibration are much higher, 

 beino- in two good instances, which I measured, 31° 15' and 

 35°. Marked features are the indistinctness of the prismatic 

 cleavage, and a somewhat marked ver}^ closely placed striation, 

 which seems to represent a separation rather than a cleavage. 

 It seems to me that this mineral stands in the same relation 

 to hornblende that diallage does to augite. It is more sub- 

 ordinate to the hornblende than both are to the magnesia- 

 iron-mica. 



The dark-coloured masses looking like included fragments 

 of some rock which are very common in the Noyang quartz- 

 mica-diorites, I found to be crystallisations of, mainly, the 

 more basic minerals. I observed the amphibole to be well 

 crystallised, frequently twinned, and strongly polychroic. 

 The prismatic angles measured in several cases were precisely 

 124° 30', taking the mean of several readings. The angles 

 formed by the plane of vibration I found in the highest 

 to be 22°. 



Felspars. The felspars of this rock are characterised by 

 being almost, if not all, plagioclase. They are well crystal- 

 lised, excepting where they have become adapted during 

 growth to the form of some mica or amphibole crystal. The 

 twinning takes place according to the Albite, and also to the 

 Carlsbad law. Sometimes the former alone, but more fre- 

 quently both conjoined. In some crystals one half of the 

 Carlsbad twin is simple, while the other is compound. 

 Besides these twinned crystals there are some lesser ones 

 which are not twinned. Of these some may be orthoclase, 

 but I found only one which had the optical characters of 

 that felspar. 



The mean of a number of measurements of the angle 

 formed by the plane of vibration gave the following: — 



OP (001) oo Poo (100) ... oo Poo (010) 



5° 30' ... 15° 45' 



