PLATE xx. 



FIG. I. 



PLAGIOCLASE-HORNBLENDE ROCK. 

 (EPIDIORITE.) 



roKTION UK SCOUKIK DVKK. 



Magnified 25 diameters. Ordinary light. 



The minerals represented are plagioclase (10), honil.)lende (S), and titaniferous 

 iron ore (5). 



The plagioclase is perfectly colourless and occurs as aggregates of irregular grains, 

 which are in many cases entirely destitute of lamellar twinning. That the original 

 plagioclase has undergone complete molecular reconstruction is proved by the absence 

 of the characteristic lath-shaped sections, by the colourless character of the secondary 

 felspar, and by the occurrence of hornblende-needles as inclusions in the felspar. 

 The original pyroxene in this rock has been replaced by green hornblende. 



The hornblende occurs as irregular and often extremely ragged grains. These 

 sometimes contain inclusions of a colourless mineral, presumably quartz (8a). It also 

 occurs in the needle-like form (actinolite), and in this condition is found scattered 

 through the secondary aggregates of felspar. 



The micro-structure is entirely different from that represented in plate XIX., 

 Fig. I. The boundaries of the hornblende and plagioclase are not the same as those 

 of the original pyroxene and plagioclase. The molecular rearrangement has been 

 accompanied by the complete obliteration of the micro-structure of the original rock. 

 Plagioclase-hornblende rocks of the type represented in this figure (epidiorites) are 

 extremely common in certain districts. We have no evidence at present to show that 

 they can be produced in any other way than by the metamorphosis of plagioclase- 

 pyroxene rocks, Compare the micro-structure of this rock with that of the Crousa 

 Down Gabbros. (Plate XVIII., Fig. 2). 



FIG. II. 

 HORNBLENDE SCHIST. 



roMATKh rouTiox OF SCOURII-: I>VKK. 

 Magnified yO diameters. Ordinary light. 



The minerals represented in this figure arc the same as those in the last. They 

 are now arranged, however, in parallel bands. The colourless mineral is certainly in 

 most if not in all cases secondary felspar. The cloudy patches (lOa) may represent 

 portions of the original felspar which have not undergone reconstruction. 



The titaniferous iron ore is arranged in lines parallel with the schistosity and is 

 generally associated with granules of sphene. The hornblende shows a rough kind of 

 orientation which is best seen by rotating the section over the polarizer. Most of the 

 grains appear a deep green when the short axis of the polarizer is parallel with the 

 plane of schistosity, and a pale yellowish green when it is at right angles to this plane. 



It will be observed that the micro-structure of the original igneous rock is bore 

 replaced by that of a crystalline- -rbist. 



