576 



KEPORT OF NATIONAL MUSEUM, 1890. 



name keratophyr has been given by Giiinbel to a quartzose or quartz- 

 free porphyry containing a sodium-rich alkaline feldspar. (Specimens 

 70609, Minnesota; 70194 and 70195, Bavaria; 70186, Hartz Mountains; 

 and 70564, Bonielo, Norway.) 



So far as can be at present judged, these rocks are much more re- 

 stricted in their occurrence than are the quartz porphyries. 



The following localities and varieties are represented: 



Andlau, Vosges Mountains, Germany, 36304 ; Hohwald, Vosges Mountains, 36303 ; 

 Katzenellnogen, Nassau, Germany, 36302 ; Oldenwald Hesse, Germany, 36301 ; 

 Christiania Fjord, Norway, 1680; near Christiania, Norway (orthophyr), 70413; 

 Tyvehomen, near Christiania, Norway (rhomb-porphyry), 1660, 34381, and 

 36456; Pigeon Point, Minnesota (Keratophyr), 70609; Torkel in the Fichtelge- 

 birge, Bavaria (Keratophyr), 70194; Bavaria, 70195; Elhingerode, Hartz Moun- 

 tains, Germany, 70186 ; Bammelo, Norway (Keratophyr), 70564. 



4. The trachytes. 



Trachyte from the Greek word rpayoq, rough, in allusion to the char- 

 acteristic roughness of the rock. The term was first used by Hauy to 

 designate the well-known volcanic rocks of the Drachenfels on the 

 Rhine. 



Mineral composition. — Under the name of trachyte are comprehended 

 by Professor Rosenbusch those massive Tertiary and post-Tertiary 

 eruptive rocks, consisting essentially of sanidiu 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 

 plagioclase, tridymite, apatite, sphene, and magnetite, more rarely 

 olivine (specimen 72988 from the Isle of Ischia); sodalite (specimens 

 36320, 36321, and 72994 from the Isle of Ischia, and specimens 73013, 

 73014, and 73017 from near Naples, Italy) ; hnmite (specimen 36331 from 

 Monte Somma) ; hauyne (specimens 36315 to 36317 from the Laacher 

 See), and mellilite (specimen 72997 from Isle of Ischia). 



Chemical composition. — The following shows the range in chemical 

 composition of these rocks (I) being that of the trachyte of Game Ridge, 

 Colorado (see specimen 70605) and (II) that of the La Guardia stone, 

 (See specimens 38788 and 73004). 



IT. 



Silica 



Alumina 



Ferric oxide 



Ferrous oxide 



Manganese oxide 



Lime 



Magnesia 



Potash 



Soda 



Ignition 



Phosphoric acid (P 2 6 



Total 



Per cent. 



66.03 



18.49 



2.18 



0.22 



Trace. 



0.96 



0.39 



5.86 



5.22 



0.85 



0.04 



rcr cent. 



56. 0D 



26.09 



1.53 



Trace. 

 3.41 

 2.70 

 6.49 

 3.38 

 1.05 



100. 24 



100. 74 



