DETAILED DESCRIPTION OF SLIDES. 129 



turbed by the formation of decomposition-products. Some of the augites 

 are almost unattacked, and show thoroughly characteristic cleavages, ex- 

 tinctions, etc. Others are partially converted to chlorite, and yet others 

 are wholly replaced by the uniaxial, dichroitic, green mineral. Some of 

 the pseudomorphs are partially converted into epidote, the characteristic 

 prismatic spruuts of which may be seen penetrating the chlorite. A fine 

 example is illustrated in Fig. 5, Plate II. In some other cases the degen- 

 eration to epidote and to calcite is going on in the same chloritic pseudo- 

 raorph. There were originally one or two small hornblendes in this slide, 

 now wholly converted into epidote. The feldspars seem to be labradorite. 

 Crown Point Ravine is the best of all the "propylite" localities, and the 

 specimen is an excellent representative of the rocks which have received 

 this name. A portion of the slide is very faithfully illustrated in Fig. 31, 

 Plate V. 



Slide 428 500 feet southeast of Sutro Tunnel air-shaft. 



Specimen with peculiar augites. — This Is a black rockwithau uneveu fracturc, 

 and a luster both vitreous and resinous Under the microscope it is seen to 

 be a fine augite-andesite with more augite than usual, and no hornblende. 

 The augite is of the common color and slightly dichroitic. One crystal 

 shows the uncommon phenomenon of multiple twinning, in which the surface 

 of composition of a portion of the lamellae is decidedly irregular. This 

 augite is illustrated in Fig. 16, Plate III. The large feldspars give angles 

 of extinction corresponding to labradorite; the microlites correspond to 

 oligoclase, and many of them show a tendency to fibration at the ends. 

 The large feldspars contain inclusions of glass and microlites of augite. 

 There are a few brown apatites and some colorless ones in the slide. The 

 groundmass has the well-known felted appearance in some portions and 

 shows fluidal arrangement in others. It contains a considerable amount of 

 isotropic glass. 



Slide 31. Hutro Tunnel, 10,055 feet from entrance. 



Specimen with unusual chlorite pseudomorph. — This is an Ordinary augite-audesite 

 in a somewhat decomposed condition, which most likely carried a little 

 9 c L 



