590 DESCRIPTIVE GEOLOGY. 



SECTION VII. 



REGION NORTH OP THE HUMBOLDT RIVER. 



BY S. F. EMMONS. 



Mallakd Hills. — The Mallard Hills form the southern continuation 

 of a high range to the northward, whence comes the principal supply of 

 water to Bishop's Creek, the main branch, properly speaking, of the Hum- 

 boldt Eiver. The higher portions of this range, as shown by the fragments 

 in the wash of the streams, are formed of similar sedimentary materials to 

 those of the Tucubits Mountains. The Mallard Hills, which are low, flat, 

 rounded ridges, are made up almost entirely of flows of rhyolites, which 

 are quite distinct in character from any observed in other portions of the 

 region explored. 



The rhyolite from Deer Canon, the northern point of these hills, is a 

 lavender-colored rock, which weathers into thin slabs, or laminse, generally 

 from half an inch to an inch in thickness. It resembles in many respects 

 the quartziferous trachytes of the Elk Head Mountains. In a light-gray 

 felsitic groundmass, it contains large crystals of sanidin and rounded grains 

 of quartz, often as large as a pea, which are cracked in the interior and are 

 surrounded by a ring of altered groundmass; it has, however, no mica or 

 hornblende. Under the microscope, the groundmass is seen to be made up 

 of a distinct aggregation of colorless plates, and black and colorless grains, 

 while the ring which surrounds the larger crystals is an extremely fine, 

 granular modification of this groundmass. 



The rhyolites which form the main peak of these hills are generally of 

 dark reddish-brown color. In some cases, they have the same laminated 

 habit as those of Deer Canon, and, like them, have something of a trachytic 

 look. They contain also large crystals of quartz and sanidin in a felsitic 

 groundmass, and have likewise apparently no mica or hornblende. They 

 are sometimes extremely porous, the druse-like cavities being lined with 

 botryoidal concretions of black glass and of chalcedony. Under the micro- 



