484 THE CRYSTAL FALLS IE0N-BEARIl!5^G DISTElCT. 



is often obscured by the abundance of decomposition products. Under low 

 powers of the microscope, however, it can nearly always be detected. In 

 a few of the finer-grained varieties, phenocrysts of plagioclase are occasion- 

 ally met with. They are clouded hj inclusions of biotite flakes and shreds 

 of hornblende and by tiny particles of a kaolinitic or sericitic mineral. 

 From their composition and structure, it is clearly evident that the intrusive 

 greenstones, whether massive or schistose, are altered j)hases of diabase or 

 of diabase-porphyrite. 



The dark-green chlorite-schist referred to as occurring in the edge of 

 one of the greenstone masses is a chloritic biotite-schist spangled with large 

 flakes of a light-colored mica. The rock consists of biotite, chlorite, musco- 

 vite, quartz, and rutile. The biotite is in broad thin plates, ai-ranged 

 approximately parallel, and embedded in a mass of chlorite, tlie g-reater 

 portion of which is a greenish-brown variety that looks as though it may 

 have been derived from hornblende. A smaller portion of the chlorite is in 

 light-green plates, like the chlorite so frequently found in chlorite-schist. 

 The quartz is in small rounded grains exhibiting strain shadows, scattered 

 here and there through the chlorite and between the biotite plates. It is 

 much more abundant in some portions of the rock than in others, forming 

 bands rich in quartz, l^etween others in which very little of this mineral is 

 present. The rutile is in large quantity. It constitutes large greenish- 

 yelloMf grains. Some of these are rounded forms, others are prismatic 

 crystals measuring 0.08 mm. to 0.12 mm. in length, while still others are 

 clearly defined elbow twins. They occur everywhere throughout the slide, 

 but are rare in the quartz. They are most abundant in the chlorite and in 

 the large plates of light-colored mica that have been mentioned as character- 

 istic features of the hand specimens. These have all the properties of mus- 

 covite. They lie indiscriminately among the other components, irrespective 

 of the schistosity of the rock, and contain very few inclusions, with the 

 exception of the rutile grains. The lines of biotite, to the 'arrangement of 

 which the rock owes its schistosity, do not bend around the muscovite as 

 they do around the eyes in an augen-gneiss, but they continue their courses 

 lap to the edge of the muscovite grain, and there abniptly stop. From these 

 facts it is clear that the muscovites have originated since the rock containing 

 them was rendered schistose. As in the case of many other secondary 

 minerals, it appears that these were produced from the components of the 



