DETAILED DESCRIPTION OF SLIDES. 103 



posed from the periphery of the hornblende section. The former supposi- 

 tion is altogether the more probable. A portion of the epidote does not 

 show the usual crystalline structure, but forms a mass of small grains or 

 scales, of which so many are superimposed upon one another in the thick- 

 ness of the section, as to present perfect aggregate polarization; indeed it 

 is difficult to detect a difference between these masses in polarized light and 

 natural light. The change of the edges of the hornblendes to quartz has 

 been accompanied by the separation of minute particles of a whitish opaque 

 material of unknown character, and further by the formation of black 

 opaque particles which can hardly be anything else than hematite or mag- 

 netite. These particles are arranged in lines parallel to the crystal edges, 

 and now surround many of the interior masses of epidote with a black 

 border. This is interesting as evidence that the black border of decomposing 

 hornblendes is sometimes a secondary formation. 



The slide contains a number of augites, some of them in very well 

 defined octagonal cross-sections. The presence of this mineral associated 

 in diorites with hornblende which was in all probability dense, is unusual 

 and interesting Like the hornblende, the augite has been completely con- 

 verted into chlorite, but the change from chlorite to epidote has begun in 

 only one or two cases. The augite is sometimes also surrounded with a 

 black border. Some of the apatites are dark brown and strongly dichroitic. 

 In all except a single case the outer edge is much more deeply colored 

 than the center, but in one instance this order is reversed. Many ordinary 

 colorless apatites are also present. The feldspars are tri clinic; little more, 

 however, can be said of them, for they are much decomposed, and filled with 

 products of decomposition. The same is true of the groundmass, in which 

 secondary quartz and calcite, veins and patches of chlorite, and grains of 

 epidote greatly obscure the original structure, but it is still apparent that 

 it was granular and not microlitic. 



Slide 233. Head of Opbir Ravine. 



This slide is from the same locality and the same cropping as 281, 

 but from another specimen. In addition to the principal features of that 

 slide, it shows unmistakable mica sections, which have undergone precisely 



