266 GEOGRAPHICAL DISTRIBUTION OF TRACHYTE. 



Siebengebirge ; it is associated with augite in the trachytes of Krem- 

 nitz, of Alsberg in the Rhon, and at Dernbach near Montabaur, and 

 Castelnuova in the Euganean Hills ; it is associated with biotite in 

 some trachytes of the Siebengebirge, and in other trachytes horn- 

 blende is absent. 



Augite may occur in microscopic or macroscopic crystals, which may 

 be well defined and regular, or irregular, or in columns or grains. 

 Twin crystals are frequent, as are fractured crystals. The inclusions 

 comprise glass, magnetite, and microliths of apatite. According to 

 the position of the cleavage so is the colour of the crystal. It is 

 often green when the axis is parallel to the short diagonal of the 

 nicol, and reddish brown when the axis is perpendicular to it. 

 Augite is not so liable to decomposition as hornblende or 

 biotite. 



Biotite is not known in the ground mass of trachyte. It occurs 

 in thin hexagonal plates, which are often surrounded with magne- 

 tite ; the colour is deep brown, but exceptionally may be blood-red, 

 as in the Yallee de la Cour in the Auvergne, at Capo Negro in Ischia, 

 and at Mocsar in Hungary. Biotite is also found in Italian trachytes, 

 at Yenda di Teolo, and Monte Zacon in the Euganean Hills. In the 

 Auvergne it is found in the trachytes of Mont Dore, Val de 1'Enfer, 

 Plateau de Durbieze, Ravin de la Craie, and in these French localities 

 is associated with augite. 



Apatite usually occurs in long needles with hexagonal section ; 

 but it is also found in short thick columns, of a grey tint, which 

 may incline to brown, blue, or violet. It abounds in some of the 

 bombs of trachyte at the Laacher See, and at Alsberg in the Rhon. It 

 is less abundant in the Drachenfels ; and it is found in the trachyte 

 of Dernbach near Montabaur, and near Kelberg in the Eifel. 



Magnetite is scattered in the ground mass in grains and in 

 crystals. It is often adherent to the crystals of augite or hornblende, 

 but never adheres to felspar. 



The accessory minerals are titanite, olivine, sodalite, hauyine, and 

 nosea, with quartz and tridymite; though in American trachytes, 

 quartz and olivine both occasionally become important constituents. 

 Leucite occurs sparingly in the trachyte of Arso-Stromes. 



North American Trackytes. Clarence King distinguishes four 

 important areas of trachyte, separated from each other by intervals 

 of four degrees of longitude in the region explored by the Fortieth 

 Parallel Survey. These masses occur in the Rocky Mountains, in 

 the Wahsatch range and Salt Lake region, in the Pinon and Cortez 

 ranges, and in the Virginia and Lake ranges near Pyramid Lake. 

 Several of these masses of trachyte have been forced out through 

 great fissures, which can sometimes be traced to lines of fault. 



From the hills at the foot of the "Washoe range a flood of 

 trachyte extends 40 miles to Pyramid Lake, and in this district caps 

 all the prominent hills. Zirkel remarks that the whole of this mass 

 has been ejected through a narrow dyke less than TOO feet wide, 

 which pierces propylite in the pass north of Gould and Curry mill. 



