199 



crystalline schist. The change may occur in the space of an inch, as may 

 be seen from the accompanying figure (Fig. 25). 



The specimen figured below was accidentally fractured, and a little of the 

 intervening material lost ; but the two portions are represented in their 

 natural position, and the zone of transition is fortunately preserved in the 

 upper one. A large section prepared from the zone of transition shows its 

 gradual character in a very perfect manner. 



The micro-structure of the hornblende schist is represented in Fig 2, 

 Plate XX., and Fig. 1, Plate XXI. The hornblende occurs in irregular 

 grains which possess definite optical characters, but are without external 



FIG. 25 (three-fourths natural size). 



crystalline faces. They are usually of unequal dimensions in the different 

 directions, and the corresponding axes lie roughly, but not rigidly parallel 

 to each other in the rock. The longest diameter of a grain usually cor- 

 responds approximately to the vertical axis of the crystal, and lies in the 

 plane of schistosity ; the mean diameter corresponds with the orthodiagonal, 

 and also lies roughly in the plane of schistosity ; the shortest diameter cor- 

 responds with the clinodiagonal, and lies at right angles to the plane of 

 schistosity. 



The pleochroism of the hornblende is : a = very pale greenish 

 yellow ; (3 = dark yellowish green ; 7 = rich deep green. The colourless 

 grains are partly felspar and partly quartz. It is impossible to separate the 

 two minerals without the use of convergent polarised light as the felspar 

 is usually untwinned. In many cases the nature of the colourless mineral 

 cannot be determined at all. In the less perfect schist (see Fig. 2, Plate 

 XX.) the turbid felspars may be relics of the original mineral. The opaque 

 iron ore may occur as grains in the schists or as long streaks lying in the 



