PETROGRAPHICAL NOTES. 15 



3. Carbonaceous Slate. 



These are represented by specimens from tlie Guadaloupe 

 River, one-half mile below Keal, and from various points 

 along the Gallo Canon. 



They are usually black, fine-grained rocks of decidedly 

 slaty structure; films of brown biotite, composed of minute 

 scales, occur on the stratification planes. 



Under the microscope they prove to consist of alternating 

 bands of coarser and finer quartz-feldspar aggregates, the 

 grains of which are well defined and join sharply. The 

 quartz grains show numerous fluid inclusions, partly with 

 moving bubbles. The liquid is often of a brownish color. 

 Films of chestnut-brown biotite, and also isolated minute 

 foils of the same substance are scattered through the rock. 



There is also much carbonaceous substance, usually ar- 

 ranged in streaks and bands, following the more fine-grained 

 parts of the rock; it appears as small grains of irregular, 

 often elongated shape. That this material is to great extent, 

 at least, formed by carbon, is proved by igniting the rock, 

 when it loses much of its dark color. 



In one specimen, the coarser quartz grains are filled witli 

 dark, short rutile-needles (ThonschiefernEedelchen), and also 

 contain some larger, yellow, knee-shaped rutile crystals. 



V. — Diabase. 



In the section of metamorphic slate exposed along the 

 Gallo Canon, and especially near the eastern granite con- 

 tact, there are great masses of dark, fine to middle-grained 

 diabases. They do not generally have a slaty structure, 

 and still the field observations would indicate that they 

 belong to or are intimately connected with the metamor- 

 phic series. Some distance west of the divide between 

 San Rafael Valley and Gallo Creek these rocks end with 

 quite a sharp contact towards the granite, to begin again 

 with a similar contact in the first foothills of the valley. 



