TUCUBITS MOUNTAINS. . 527 



erable development of grains of limpid quartz. These greenish breccias 

 gradually lose all appearance of a granular groundmass, the whole mass 

 seeming to be more or less felsitic, and pass into a porphyritic rhyolite, 

 having something of a brecciated structure, which is full of crystals of lim- 

 pid quarts, and carries some few feldspar crystals. In this range, it has 

 almost the appearance of a quartz-porphyry, and may possibly be of older 

 age than the rhyolites, though, as will be seen later, the same association' is 

 found with well-defined Tertiary rhyolites, of whose age there can be no 

 doubt. 



Out of the Tertiary plains, to the west of the Tucubits Mountains, rises 

 a little group of hills called Forellen Buttes, which is composed of a some- 

 what similar brecciated rhyolite, having a rather compact, grayish-drab, fel- 

 sitic groundmass, in which are large crystals of sanidin and quartz. Under 

 the microscope, this rhyolite is seen to be made up of three different mate- 

 rials: first, broken crystals of sanidin and quartz, lying close together; sec- 

 ond, fragments of dark-gray hornstone rhyolite, rich in glass; and, third, 

 bands of dull whitish rhyolite, enclosing angular fragments of quartz, which 

 envelop the other ingredients in wavy lines. 



