HANDBOOK FOR THE DEPARTMENT OF GEOLOGY. 559 



Structure. — Diorites are holocrystalline granular rocks, and are, as a 

 rule, massive, though schistose forms occur. The individual crystals 

 composing the rock are sometimes grouped in globular aggregates, 

 thus forming the so-called Orbicular diorite, Kugel diorite, or Napoleou- 

 ite, shown in specimen No. 6722 from Corsica (see also PL oxxvi). 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. 



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



Classification. — Accordingly as tbey vary in mineral composition the 

 diorites are classified as (1) diorite, in which hornblende alone is the 

 predominating accessory ; (2) mica diorite, iu which black mica replaces 

 the hornblende, and (3) augite diorite, iu which the hornblende is par- 

 tially replaced by augite. The presence of quartz gives rise to the 

 varieties, quartz, quartz augite, and quartz micadiorites. The name ton- 

 alite has been given by Vom Rath to a quartz diorite containing the 

 feldspar andesiue and very rich in black mica (specimen 363G1, from 

 Monte Tonale, in the Tyrol). Kersantite is a dioritic rock occurring, 

 so far as known, only in dikes, and consisting essentially of black mica 

 and plagioclase, with accessory apatite and augite, or more rarely horn- 

 blende, quartz, and orthoclase (specimens 36356, 36357, 36360, 70206, 

 36358, 36359 and 39039). It differs from the true mica diorite in being, 

 as a rule, of a porphyritic rather than granitic structure. Professor 

 Rosenbusch, in his latest work, has placed the kersantites, together 

 with the porphyritic diorites (Camptonites), under the head of dioritic 

 lamprophyrs in the class of dike rocks or " gangesteine." The name, 

 it should be stated, is from Kersantou, a small hamlet in the Brest 

 Roads, department of Fiuistere, France. 



The diorites were formerly, before their exact mineralogical nature 

 was well understood, included with the diabases and melaphyrs under 

 the general name greenstone (Ger. Gruustein). They are rocks of wide 

 geographic distribution, but apparently less abundant in the United 

 States than are the diabases. The lamprophyr varieties are still less 

 abundant, so far as now known, and are represented in the collection, 

 as will be noticed, by specimens from but five localities. 



The following localities and varieties are at present represented : 



Diorites proper : Near Reading, Berks County, Pennsylvania, 26474; Comstock Lode, 

 Nevada, 24151, 24154, 24005, 24007, 24042; Agate Pass, Cortes Range, Nevada, 

 21312; Virginia Range, Washoe County, Nevada, 22616; Jamestown, Boulder" 

 County, Colorado, 2G876 ; Howland, Lake County, Colorado, 68963; Buckskin 

 Gulch, Park County, Colorado, 68967 ; Madison County, Montana, 73171, 73172, 

 and 73173 ; Rio San Francisco, Brazil, 69870 ; Hesse, Germany, 36365 ; Redwitz, 

 in the Fitchelgebirge, Bavaria, 36366 ; Weinheim and Schriesheim, Baden, 36367 

 and 3669 ; Frieberg, Saxouy, 36368 ; La Fiesse, Vosges, France, 36372 ; Grissel- 

 ham, Upland Sweden, 35635. 



Orbicular diorite, kugel diorite, or napoleonite, Corsica, 67222, 36054 (polished slab). 



Mica diorite: Stewartstown, New Hampshire, 27860; near Croton, New York, 72957; 

 Comstock Lode, Nevada, 24124 ; Stony Point, Rockland County, New York, 38342; 

 Lippinhof, near Freiberg, Black Forest, Baden, 38345, 



