TRACHYTES OR SUB-ACID ROCKS. 

 Group II.— TRACHYTES OR SUE-ACID ROCKS— Continued. 



107 





Characteristics. 



3. Argllloid trachyte 



A rock of very clayey or eartliy aspect, suggestive of thick slate ; very 

 highly charged with ferritic matter, rendering it opaque in the thin- 

 nest sections; holding crystals of feldspar (orthoclase) and grains of 

 magnetite, and seldom any other macroscopic mineral. The fracture 

 is highly characteristic, there being no cleavage ; but the rock crum- 

 bles rather than splits. It is impossible to strike off thin flakes. 

 The fracture is very angular and irregular, though the ordinary 

 coarseness of trachytes is not exhibited. It is a very voluminous 

 rock in the plateaus and well distinguished. 



4. Hyaline trachyte 



Trachytes having a fluidal texture, indicative of flowing in a viscous 

 state, with very small, and sometimes few, and always poorly-devel- 

 oped crystals of feldspar. Mostly reddish or purplish ; often with a 

 brick-like texture ; sometimes foliated and resonant (clink-stone) ; 

 moderately vesicular. Often slightly quartziferous and approaching 

 the rhyolites. 



Sub-group B. — Hornblendic Trachytes. 



5. HORNBLENDIC TRACHYTE... 



This comprises most of those dark-colored varieties of coarse, harsh 

 texture, exceedingly rough, though many are less so. Hornblende 

 and magnetite are abundant, the former in well-developed prisms. 

 The feldspars are less conspicuous than in the preceding varieties, but 

 are really present in greater quantity, as shown by the microscope. 

 Plagioclase very abundant. Iron gray is the usual color. 



6. AjJGITIC TRACHYTE 



It seems doubtful whether this rock should be considered as anything 

 more than a variety of the homblendic sub-group. It is character- 

 ized by the presence of augite in place of hornblende. The varieties 

 are usually finer grained than the homblendic, and resemble more the 

 augitic andesites, to which, indeed, they are so closely related that it 

 is sometimes difficult to distinguish them. Magnetite abundant and 

 some biotite. 





7. PnONOLITE 



A rock in which nephelin takes the place of triclinic feldspar. Usually 

 contains also orthoclase and some hornblende ; resonant, foliated, and 

 in the rockmass is generally laminated in a very peculiar and strik- 

 ing manner. 





8. TRACIIYTIC OBSIDIAN 



A wholly glassy or vitreous rock, having the normal constitution of 

 trachyte. 



