580 



REPORT OF NATIONAL MUSEUM, 1890. 





Si 2 



Al 2 3 



Fe 2 3 



Fe 



Mg 



Ca 



Nai 



K 2 



H 3 



1 



66.32 

 69.51 

 61.12 

 56.07 

 56.19 

 58.33 



14.33 

 15. 75 

 11.61 

 19.06 

 16.12 

 18.17 



5.53 

 3.34 

 11.61 

 5.39 

 4.92 



0.25 



0.92 

 4.43 

 6.03 



2.45 

 2.09 

 0.61 

 2.12 

 4.60 

 2.40 



4.64 

 1.71 

 4.33 



7.70 

 7.00 

 6.19 



3.90 

 3.89 

 3.85 

 4.52 

 2.96 

 3.20 



1.61 

 3.34 

 3.52 

 1.24 

 2.37 

 3.02 



1. 13 



2 .-. 





3 



4.35 



4 



0.99 





1 03 



6 



0.76 







1 Dacite from Kis Sibes Transylvania. 



2 Dacite from LassensPeak, California. 



3 Hornblende andeaite from hill north of Gold Peak, Nevada. 



4 Hornblende andesite from Bogoslof Island, Alaska. 



5 Hypersthene andesite, Buffalo Peaks, Colorado. 



6 Augite andesite from north of American Flat, Washoe, Nevada. 



Structure. — To the unaided eye the andesites present as a rule a com- 

 pact, amorphous, often rough and porous groundmass carrying porphy- 

 ritic feldspars and small scales of mica, hornblende, or whatever may 

 be the prevailing accessory; pumiceous forms are not uncommon 

 (specimens 35516 and 36979). Under the microscope the groundmass 

 is found to vary from clear glassy through microlitic forms to almost 

 holocrystalline. The minerals of the groundmass are feldspars in 

 elongated microlites, specks of iron ore, apatite in very perfect forms, and 

 one or more of the accessory ferro-magnesian minerals. The porphy- 

 ritic constituents present well developed crystalline outlines, or having 

 once been fully developed have suffered from the corrosive action of 

 the molten magma, the feldspars being rounded, and the hornblendes 

 displaying a characteristic black border. A pronounced flow structure 

 is usually developed. The micro-structure as seen by a low power is 

 well shown in the colored transparency in the window number (39080). 



Colors. — The prevailing colors are some shade of gray, greenish or 

 reddish. 



Classification and nomenclature. — Specific names are given dependent 

 upon the character of the prevailing accessory. We thus have : 



Andesites with quartz— Quartz andesites or dacites 



Audesites in which, hornblende prevails=j5ro/ - H/>/erc(Ze andesites. 



Andesites in which augite i>reYa l i\s=Augite andesites. 



Andesites in which hypersthene prevails= Hypersthene andesites. 



Andesites in which mica prevails=Mtca andesites. 



The glassy varieties are often knowu as hyaline andesites. The name 

 propylite was given by Eichthofeu to a group of andesitic rocks preva- 

 lent in Hungary, Transylvania, and the western United States (speci- 

 mens No. 36414 and 35323 from Colorado and Nevada and 36481 from 

 Hungary), but these rocks have since been shown by Wads worth and 

 others to be but altered andesites, and the name has fallen into disuse. 



The following varieties and localities are represented: 



Quartz andesite : Dacite. Lassen's Peak, California, 70597 ; State of Mexico, Mexico, 

 37710; Nagy Ag, Transylvania, 34587 and 34575; Schemnitz, Hungary, 70180 and 

 70181 ; Kis Lebes, in the Siebengebirgc, 30391. 



