PORPHYRTTE. 85 



microscope detects also augite, ortlioclase, zircon, titauite, magnetite, hema- 

 tite, and apatite. 



PORPnYRITE. 



As compared with the quartz-porphyries, tlie t}'pe rocks of this chiss 

 are distinguished at first gk^nce by a great predominance of basic siHcates 

 (hornblende or biotite), by a comparative rareness of quartz, and by their 

 rather younger field habit, as shown by the marked conchoidal fracture 

 and generally fresher appearance. For the latter reason it was at first 

 thought in the field that they might possibly be of Tertiary age, but the 

 foct that they are folded and faulted with the inclosing Paleozoic rocks, as 

 well as their internal structure, proves them to be, like the quartz porphy- 

 ries, of Secondary age. In their manner of occurrence they are also distinct 

 from the latter rocks, in that they do not form large bodies, neither dikes 

 nor intrusive sheets being as a rule over twenty feet in thickness. The 

 former often occur in the form of interrupted dikes; the latter, on the 

 other hand, while occasionally crossing from bed to bed, have a most 

 remarkable extent in one general horizon as compared with the thickness 

 of the sheet. Although subordinate in amount to the quartz porpliyries, 

 these rocks occur with so many variations of internal structure and compo- 

 sition that they aff'ord a complete series, including almost all the possible 

 varieties of the type, and a complete description and classification made by 

 Mr. Cross from a lithological point of view will be found in Appendix A. 

 Only the general features of the rocks will therefore 1)e given here. 



The typical rock, both in composition and manner of occurrence, may 

 • be taken as that which occurs interbedded in the Paleozoic beds along the 

 cliff sections on either side of Mosquito gulch. A photograph of a hand 

 specimen of this rock is reproduced in Plate VII, Fig. 2, which gives some 

 idea of its general appearance; it is a rather dark greenish-gray rock, witli 

 dark weathered surface and clean conchoidal fracture. The most promi- 

 nent macroscopical constituents are well defined prisms of dark hornblende 

 and small, white, opaque crystals of plagioclase. The microscope detects 

 some biotite both among the porphyritic constituents and in the ground- 

 mass, and both ortlioclase and quai'tz in the groundmass No glass and but 

 few fluid inclusions are found. 



