DETAILED DESClllPTION OF SLIDES. 109 



amount of plagioclase present, which seems from the character of the band- 

 ing and the angles of extinction to be oligoclase. There is certainly less 

 plagioclase than unstriated feldspar. The feldspars contain fluid inclusions. 



The quartz is present for the most part in inacroscopical grains, wliich 

 are bounded in part by crystalline outlines and in jjart by curved lines. 

 One large mass appears to have been broken, and a narrow line of ground- 

 mass separates the parted edges. In many cases deep sinuous bays of 

 groundmass penetrate the quartz, and patches of groundmass are sometimes 

 surrounded by it. A considerable number of inclusions are sparsely scat- 

 tered through the quartz. Of these the fluid inclusions are somewhat in 

 excess of the glass. The glass inclusions, which all show bubbles, are rather 

 large, and often penetrate the slide, so as to extinguish light between crossed 

 Nicols. The glass is colorless ; its shape is often dihexahedral. The fluid 

 inclusions are smaller but very characteristic, many of them having exceed- 

 ingly active bubbles. None of them appear to be carbonic acid. Although 

 there are comparatively few inclusions in these quartzes, they are so dis- 

 tributed that both kinds are often in the field of a Hartnack No. 7 objective 

 at once. 



The hornblendes are entirely decomposed to chlorite. They have a 

 black border, which does not appear to me to have resulted from weather- 

 ing of the hornblende substance. The mica too is entirely decomposed. 



There is no large quantity of iron ore, and its character is somewhat 

 indefinite. It is present in irregular forms, but there are no sections with 

 the characteristic cleavages of ilmenite; neither is there any ferric oxide 

 accompanying the mineral. The groundmass shows traces of fluidal struct- 

 ure, and in places is pseudo-spherolitic. There is no base, but as the ground- 

 mass is impregnated with decomposition products, calcite, quartz, and minute 

 grains of epidote, it is probable that any glass that may have been present 

 would be devitrified. There are a few poor zircons and colorless apatites 

 in the slide. The rock is shown as it appears under the microscope in 

 Fig. 27, Plate IV. 

 Slide 304. 1,000 feet south by west of railroad tunnel above Red Jacket. 



A second example. — This rock is Hot distinguishable macroscopically from 

 that last described (slide 354). Microscopically it is also very similar. The 



