762 DESCEIPTIVE GEOLOGY. 



five, but in many cases the fifth side will be much longer than the other 

 four, with a slightly curving outline and a tendency to develop a sixth side 

 The four-sided figure would seem to be the least common. Usually they 

 stand in an approximately vertical position, that is, above an angle of 60°, 

 and are frequently diagonal to the beds in which they are developed, and, 

 as might be expected, the steepest columns are, in general, near the crest 

 where the slope of the ridge is greatest. The tendency to columnar struct- 

 ure shows itself in various degrees of perfection from the symmetrical prism 

 to a single set of parallel planes diagonal to the bedding of the rock. The 

 most ]3erfect prismatic forms are found near the summit, becoming less and 

 less sharply developed farther down the slopes. As these columns readily 

 break away and tumble down, the mountain-slope presents a huge debris 

 pile of shattered prisms of all lengths and sizes. 



On Plate XXIII is shown a northern and eastern view of the Karnak 

 Ridge. Although columnar trachytes and rh3^olites are by no means 

 unknown, they are a much less common occurrence than the basaltic forms, 

 and it is doubtful if there is any place in the world where they are developed 

 on a grander scale than in the Montezuma Range; certainly not in Nevada, 

 where rhyolitic modes of occurrence are so richly displayed. 



The lithological characteristics of this rock are somewhat peculiar, con- 

 sisting of a micro-crystalline groundmass, richer in secreted minerals than is 

 usually found of the rhyolites of the Montezuma Range, and with a coarse 

 rough texture and habit like many trachytes. Brilliant but small crystals 

 of sanidin, black biotite, and long blades of dark hornblende are easily 

 recognized by the eye, while scattered ' through the groundmass are occa- 

 sional translucent quartz-grains. Under the microscope may be seen 

 considerable apatite and plagioclase, but, what is somewhat singular, no 

 quartz. The occurrence of so much plagioclase and the absence of micro- 

 scopical, in the presence of macroscopical, quartz-grains, are marked features 

 of some trachytes, but rare in rhyolites; while, on the other* hand, the 

 groundmass presents structural phenomena characteristic of rhyolitic types. 

 Fragments of granite enclosed in the groundmass of the prisms have been 

 observed. 



The West Fork of Bayless Canon exposes a great variety of bril- 



