72 



ROCKS FORMEB THROUGH IGNEOUS AGENCIES 



name heraiophyr was given by Giimbel to a quartzose or quartz- 

 free porphyry containing a soclium-rich alkaline feldspar. So 

 far as can be at present Judged, rocks of this type are much 

 more restricted in their occurrence than are the quartz porphyries 

 already described. 



(3) THE TRACHYTES 



Trachyte, from the Greek word rpax^s, rough, in allusion to 

 the characteristic roughness of the rock. The term was first 

 used by Hatiy to designate the well-known volcanic rocks of the 

 Drachenfels on the Rhine. 



Mineral Composition. — Under the name of trachyte are com- 

 prehended those massive Tertiary and post-Tertiary lavas, con- 

 sisting essentially of sanidin with hornblende augite or black 

 mica, and which may be regarded as the younger equivalents of 

 the quartz-free porphyries. The common accessory minerals 

 are pJagioclase, tridymite, apatite, sphene, and magnetite, more 

 rarely, sodalite, hauyne, and mellilite. 



Chemical Composition. — The following analyses show the 

 range in chemical composition of these rocks, I being that of 

 a trachyte from Game Ridge, Colorado, and II that of one from 

 San Pietro island, Sardinia. 



Chemical Composition op Teachyte 



COUfSTlTUENTS 



Silica (Si02) . • . . 

 Alumina (AI2O3) . . 

 Perric oxide (FegOs) . 

 Manganese oxide (MnO) 

 Lime (CaO) .... 

 Magnesia (MgO) . . . 

 Potash (K2O) .... 

 Soda (Na20) .... 

 Ignition (H2O) . . . 

 Phosphoric acid (P2O5) 



Total 



100.24 % 



II 



66.03% 



66.09% 



18.49 



26.09 



2.18 



. . a 



Trace 



Trace 



0.96 



3.41 



0.39 



2.70 



6.86 



6.49 



6.22 



8.38 



0.85 



1.05 



0.04 







100.74% 



Structure. — In structure the trachytes are rarely granular, 

 but possess a fine, scaly or microfelsitic ground-mass, rendered 

 porphyritic through the development of scattering crystals of 



