THE POEPHTBTTES AND ANDESITES 



79 



quantities, the rock is designated as simply porphynU. The 

 porphyrites are wide-spread rocks, very characteristic of the 

 later Palaeozoic formations, occurring as contemporaneous lava 

 flows, intrusive sheets, dikes, and bosses. 



(3) THE ANDESITES 



The name Andesite was first used by L. Von Buch in 1835, to 

 designate a type of volcanic rocks found in the Andes Moun- 

 tains, South America. 



Mineral Composition. — The essential constituents are soda- 

 lime feldspar, together with black mica, hornblende, augite, or 

 a rhombic pyroxene, and in smaller, usually microscopic pro- 

 portions, magnetite, ilmenite, hematite, and apatite. Common 

 accessories are olivine, sphene, garnets, quartz, tridymite, anor- 

 thite, sanidin, and pyrite. 



Chemical Coinposition. — The composition of the andesites 

 varies very considerably, the quartz-bearing members naturally 

 showing much the higher percentage of silica. The following 

 table shows the composition of a few typical forms : 



Chemical Composition' of Andesite 



Constituents 



Silica (SIO2) . . . 

 Alumina (AI2O8) . 

 Ferric oxide (FeaOg) 

 Ferrous oxide (FeO) 

 Magnesia (MgO) . 

 Lime (CaO) . . . 

 Soda(Na20). . . 

 Potash (K2O) . . 

 Water (H2O) . . 



66.32 % 

 14.33 



5.53 



0.25 



2.45 



4.64 



3.90 



1.61 



1.13 



100.16% 



II 



69.61% 

 16.75 

 3.34 



• a « • 



2.09 

 1.71 

 3.89 

 3.34 



99.63 % 



III 



61.12 % 



11.61 



11.64 



• • • • 



0.61 



4.33 



3.85 



3.52 



4.35 



101.03% 



IV 



56.07 % 

 19.06 



5.39 



0.92 



2.12 

 7.70 

 4 52 

 1.24 

 0.99 



98.01 % 



66.19% 

 16.21 



4.92 



4.43 



4.60 



7.00 



2.96 



2.37 



1.03 



99.62 



YI 



58.33% 

 18.17 



6.03 

 2.40 

 6.19 

 3.20 

 3.02 

 0.76 



98.10% 



I. Baeite from Kis Sebes, Transylvania. II. Dacite from Lassens Peak, 

 California, III. Hornblende andesite from Mil north of Gold Peak, 

 Nevada. lY. Hornblende andesite from Bogoslof Islandj Alaska. V. Hy- 

 persthene andesite, Buffalo Peaks, Colorado. YI. Augite andesite from 

 north of American Flat, Washoe, Nevada. 



Structure. — To tlie unaided eye the andesites present as a 

 rule a compact, often rough and porous ground-mass carrying 



