THE DIOEITE-ANDESITE GROUP 



77 



hornblende or black mica. The common accessories are mag- 

 netite, titanic iron, orthoclase, apatite, quartz, augite, black mica, 

 and pyrite, more rarely garnets. Calcite, chlorite and epidote 

 occur as alteration products. 



Structure. — Diorites are holocrystalline granular massive 

 rocks. The texture is, as a rule, fine, compact, and homogeneous, 

 and its true nature discernible only with the aid of a microscope ; 

 more rarely porphyritic forms occur as in the camptonites. The 

 individual crystals are sometimes grouped in globular aggregates, 

 thus forming the so-called orbicular dionte, hugel diorite, or 

 napoleonite from Corsica. (Fig. 1, PL 7.) 



Colors. — The colors vary from green and dark gray to almost 

 black. 



Chemical Composition. — The following table shows the wide 

 range in chemical composition found in rocks commonly grouped 

 under this head. 



Chemical Composition of Diorite 



Constituents 



Silica (Si02) . . . 

 Alumina [Alfi^) . . 

 Ferric iron (Fe^Og) . 

 Ferrous iron (FeO) . 

 Lime (CaO) .... 

 Magnesia (MgO) . . 

 Potash (Kfi) . . . 

 Soda (Na,0) . . . 

 Phosphoric acid (P2O5) 

 Carbonic acid (CO2) . 

 Water (Bfi) . • . 



I 



n 



ill 



IV 



67.54% 



61.75% 



56.71% 



50.47% 



17.02 



18.88 



18.36 



18.73 



2.97 



0.52 



• • • • 



4.19 



0.04 



3.52 



6.45 



4.92 



2.94 



3.54 



6.11 



8.82 



1.51 



1.90 



3.92 



3.48 



2.28 



1.24 



2.38 



3.56 



4.62 



3.67 



3.52 



4.62 



[0.55 



4.46 



* «» • « 



0.58 



43.50% 



17.02 



13.68 



8.15 

 6.84 

 2.84 

 2.84 



4.35 



I. Quartz-miea diorite: Electrie Peak, Yellowstone Park (J. P. Iddings). 

 II. Diorite: Penmaen-Mawr, "Wales (J. A. Phillips). III. Diorite: Corn- 

 stock Lode, Nevada (40th Parallel Survey). IV, Augite diorite: Custer 

 County, Colorado (Whitman Cross). "V. Porphyritic diorite (camptonite) : 

 Pairhaven, Vermont (J. P. Kemp). 



Classification. — Accordingly as they vary in mineral compo- 

 sition the diorites are classified as (1) diorite^ in whicli horn- 

 blende alone is the predominating accessory; (2) mica diorite, 

 in which black mica replaces the hornblende, and (3) augite 

 diorite, in which the hornblende is partially replaced by augite. 



