268 MINERAL STRUCTURE OF PHONOL1TE. 



little mica, a comparatively large amount of augite, and some olivine. 

 The rock forms rounded dome-shaped hills and sharp cones. 



On Skellig's Ridge a dyke of trachyte from 20 to 50 feet thick 

 rises out of the sandstone to a height of 150 feet, and extends with 

 vertical walls and horizontal columnar structure for five or six miles, 

 with only one break. 



The walls are pitted wherever the grains of quartz have weathered 

 from its surface, and they here occur in double six-sided pyramids. 

 This rock combines the constituents of basalt and rhyolite. 



Many American trachytes are remarkable for the quantity of their 

 augite. Thus in the hills between Sheep Corral Canon and Wads- 

 worth, the trachytes contain abundance of sanidine, associated with 

 pale-green augite, brown hornblende, and some plagioclase in a ground 

 mass of colourless crystals and microliths. At Truckee Ferry the 

 augite prisms are grouped in oval, imperfectly radial accumulations, 

 and another example of augite trachytes is seen between Green River 

 and Bitter Creek. 



Near the Wahsatch Range so little of the ground mass is left that 

 the trachyte has frequently a granitoid aspect. 



Near Salt Lake City tridymite and quartz occur together in the 

 purple trachyte ; and the hornblende crystals contain fluid inclusions 

 with moving bubbles as well as simple gas cavities. The augite 

 crystals are here free from the magnetite so characteristic of basalts. 

 In the upper valley of Susan Creek the trachyte contains granular 

 aggregations of rose-coloured garnets and hexagonal grains of hauyine. 

 Trachytes vary in colour, but are commonly tints of green, grey, and 

 purple, and make transitions on the one hand to the tints of the 

 rhyolites, and through the black varieties approximate to the augite 

 andesites and basalts. 



Trachyte eruptions are usually free from lines of bedding, by 

 which the material may be traced up to the vent from which it 

 flowed. In the North- American region it may have either rhyolite 

 or basalt resting upon it. 



Phonolite. 



Phonolite is defined as a quartzless rock, consisting of a com- 

 bination of sanidine, with nepheline and leucite. Hence it is essen- 

 tially a nepheline or leucitic trachyte, and has the same relation 

 to trachyte that nepheline-syenite bears to syenite. Phonolite is 

 probably a volcanic representative of nepheline-syenite. There is no 

 known combination of orthoclase and leucite in the older series of 

 rocks. Though these minerals predominate in phonolite, pyroxene 

 and amphibole are essential though subordinate constituents, and 

 hauyine is often abundant. Titanite, apatite, and magnetic iron are 

 diffused in the ground mass. Plagioclase is absent in most true 

 phonolites, and, when present, is as rare as in the mica-syenite, 

 called minette. Sanidine occurs in long rod-shaped crystals, which 

 may form the greater part of the rock, or occur sparingly between the 



