440 EEPORT ON NATIONAL MUSEUM, 1886. 



about 25 miles from the Red Sea and 85 miles from aneient Captos 

 (now called Kypt). Rocks of this class, though in no way comparable 

 from the standpoint of beauty, have been described by Hawes* as oc- 

 curring in New Hampshire at Camptou Falls, North Lisbon, Dixville, 

 and Dixville Notch ; a mica diorite is also described as occurring at 

 Stewartstown. None of these are put to any practical use. A dark 

 gray granitic appearing diorite of variable texture occurs near Read- 

 ing, Berks County, Pa., which may answer for rough construction. It 

 is not a handsome stone, and is, moreover, hard to work. 



The Museum collections contain a cube of a compact light greenish 

 gray diorite, carrying quite an amount of greenish mica and plentifully 

 besprinkled with white porphyritic feldsi>ars from near El Paso, Tex. 

 This cuts to a sharp edge and acquires a good surface and polish. It 

 appears like a good stone for ordinary purposes of construction. 



A somewhat similar stone is found near Monarch, Chaffee County, 

 Colorado. 



A quartz diorite of a coarse granitic structure is found and quarried 

 at Rocklin, Placer County, Cal. The stone resembles granite in gen- 

 eral appearance and works with equal facility. 



(2) THE ANDESITES. 



Under the name of andesites is included a group of volcanic rocks of 

 Tertiary and post-Tertiary age, and consisting essentially of a triclinic 

 feldspar and hornblende, augite, or black mica. 



In structure the andesites are rarely entirely crystalline, but usually 

 present a fine densely microlitic or partly glassy groundmass. Accord- 

 ing as they vary in composition four principal varieties are recognized : 

 (1) Quartz andesite (Dacite) or andesite in which quartz is a prominent 

 ingredient; (2) hornblende andesite; (3) augite andesite, and (4) mica 

 andesite, each taking its name according as hornblende, augite, or mica 

 is the principal accessory mineral. Hypersthene andesite, or andesite 

 in which the mineral hypersthene is a leading constituent, is also com- 

 mon in many of the Western States and Territories. 



The andesites are as yet but little used for structural purposes, and 

 this largely for the same reasons as were given in the chapter on Lipa- 

 rites. Like the rhyolites they will not polish and are in no way suited 

 for decorative work. Although very abundant throughout many of the 

 Western States and Territories they have been quarried in an itinerant 

 way only at Reno and Virginia City, Nev. The rock from the latter 

 source is said to quarry easily and cut well when first taken out, and 

 to harden on exposure. The Reno andesite has been used in the con- 

 struction of the prison and a few stores at that place. 



* Geol. of New Hampshire, Vol. in, Part IV, p. 160. 



