MICRO-STRUCTURE AND MINERALOGY OF THE AMPHIBOLITE 437 



dividuals, roughly columnar in outline and from 5 to 8 millimeters in 

 length. They are sometimes intimately interlaced or grouped in bun- 

 dles. Judging from the cleavage surfaces as seen in the hand specimen, 

 one is surprised that the cleavage partings are not more prominent in 

 the thin-section. In sections normal to the vertical axis the cleavage 

 partings are well denned, but in the prismatic and pinacoidal zones, 

 while in instances easily recognized minute hair-like lines, often fading 

 out and reappearing again, may still be so fine that a higher power is 

 necessary to make this structure clear and evident. There is also a rude 

 transverse parting, which in the main should correspond in direction and 

 position to the plane of the basal pinacoid (001). The pleochroism is 

 unusually strong. It ranges from a greenish yellow parallel a to bright 

 green on b and blue on r. This is a characteristic feature of the horn- 

 blende in amphibolitic schists. 



Not infrequently the hornblende shows evidence of decomposition 

 and passage into secondary products. In most cases the decomposition 

 product is ordinary chlorite, but transitions into penninite are not un- 

 common. It can be recognized by its deep purple interference color, 

 which does not appear in any other member of the chlorite group. In 

 its passage to chlorite the typical cleavage of the original is lost and a 

 scaly or lamellar structure occurs. Very minute inclusions may be also 

 recognized. They lie in two positions — either parallel the prismatic 

 cleavage or normal to it. Occasionally larger individuals may be indis- 

 criminately scattered through their host. 



In addition to the hornblende there occurs a variable percentage of a 

 light colored to colorless silicates. A large portion of these constituents 

 is orthorhombic epidote or zoisite. The lack of extinction angle and 

 prevalence of low interference colors, not exceeding those of the first 

 order, serve to distinguish it from monoclinic epidote, which exhibits an 

 extinction angle and interference colors approaching the third order. In 

 most sections the greater part of the colorless silicate is zoisite. Two 

 cleavages are prominent, one lying parallel to the long axis (c), the other 

 a prominent transverse parting at right angles to the former and some- 

 times fading out before traversing the entire width of the individual. In- 

 clusions are present, but no systematic arrangement is prominent. They 

 are usually too small to make specific determination with any degree 

 of certainty. In a few cases, however, it is comparatively easy. The 

 peculiar pink tint and rhombic outline of faces as may be seen under a 

 high power (240) show that some of these inclusions, at least, are minute 

 garnets. They are confined to the central portion of the host. On ac- 

 count of the predominance of the optical properties of the host the iso- 

 tropic character of the garnet is completely overshadowed. In addition 



