738 DESCRIPTIVE GEOLOGY. 



together witli a considerable development of clay shales and siliceous, finely 

 laminated slates, which strike a little north of east, and dip steeply to the 

 northwest. Their lithological habit is not very characteristic, nor were any 

 fossil remains found in them, but -they have been referred for the reasons 

 already mentioned to the Jurassic formation. They are underlaid by a 

 body of syenitic gi-anite, which is exposed at the western end of the peak,' 

 while along the southeastern foot-hills are found considerable exposures of 

 diorite. In the neighborhood of this diorite body, the slates are traversed 

 by numerous mineral-bearing veins, which have a strike and dip generally 

 in conformity with the formation. 



The syenitic granite, which has been indicated by the colors of both 

 granite and syenite upon the map, is a fine-grained rock, made up of feld- 

 spar, hornblende, and mica. The feldspars are much decomposed, and 

 their crystalline character so indistinct that it is difiicult to say which is the 

 prevailing form. To the naked eye, the hornblende appears to predominate, 

 the mica, which belongs to the dark magnesian variety, occurring in large, 

 isolated, hexagonal plates, which seem to be rather an accessory constituent, 

 but under the microscope the mica is seen to rather predominate over the 

 hornblende, for which reason Professor Zirkel has classed it as a granite 

 rather than as a syenite. The diorite at the southwestern point of the hills, 

 near the granite body, is a greenish-gray, rather crystalline rock, made up 

 of plagioclase and hornblende, with some quartz. The hornblende is gen- 

 erally fibrous, and seems to be much decomposed, and, under the microscope, 

 shows remarkable phenomena of alteration. The diorite from the eastern 

 point of the hills is similar to this rock, but the plagioclase crystals are 

 larger and fresher, which give to it more of a porphyritic structure. Under 

 the microscope, it presents a beautiful instance of pseudomorphism of epidote 

 after hornblende. It also contains titanic iron. The diorite from the centre 

 of the mass is a more even-grained rock, having a less porphyritic structure, 

 which is made up mostly of triclinic feldspar, hornblende, and quartz, with 

 considerable macroscopical brown biotite. Under the microscope, the ground- 

 mass is composed almost wholly of quartz and hornblende. The quartz 

 crystals contain a great many fluid-inclusions, which are of dihexahedral 

 forms, and contain salt cubes like those found in the crystalline rocks from 



