288 IRVING. 



It is to be regarded as the final and most basic segregation of 

 the highly alkaline magma, which has produced the phonolite 

 iseries. 



8. Amphibolites. 



Under this head are classed together a large and varied group 

 of rocks whose chief constituent is a secondary hornblende, or 

 uralite. This can in some cases be traced back to an original 

 pyroxene, but at times the alteration has been complete, and we 

 have no means of determining the origin of the mineral. 



The amphibolites occur in dikes of varying width between 

 walls of Algonkian slates and schists. Whenever they are very 

 wide, the center is an unaltered core of very homogeneous char- 

 acter. Increasing schistosity can then be traced outwards, till 

 the rock passes into extremely fine, greenish mica slates and 

 phyllites. Besides these dikes others of very large and ex- 

 tremely irregular character also occur. Such is the rock in the 

 bed of Squaw Creek. 



The rock from the mouth of Fantail and Nevada Gulches 

 forms a large dike. A specimen from the center and least schis- 

 tose portion of the dike is a dense deep green rock, and, even 

 in the hand specimen, presents a slightly diabasic appearance. 

 Under the microscope it is seen to be made up of a mass of 

 large plagioclase rods with well developed boundaries. The 

 interstices are filled with a light green uralitic hornblende and 

 some augite. 



The plagioclase gives a maximum extinction angle measured 

 in sections perpendicular to albite lamellae of about i8 to 20 

 degrees. The feldspar is but little altered. The hornblende is 

 evidently a decomposition product of the augite, for it frequently 

 fills the cavities between the large fragments of that mineral, 

 which shows characteristic twinning and optical properties. The 

 augite, however, is in very small quantities comparatively, and 

 is important only in relation to the origin of the hornblende. 

 The latter is a very light greenish color, and has but a slight 

 pleochroism, from light green to yellowish green, and is most 

 frequently fibrous. It makes up the body of the rock. The 



