582 REPORT OF NATIONAL MUSEUM, 1890. 



holocrystalline diabase granular groundinass of augite, iron ores and 

 feldspars, in which are embedded porphyritic liine-soda feldspars — 

 mainly labradorite — idiomorphic augites, and at times accessory horn- 

 blende and black mica j (2) sprite, which includes the non-porphyritic 

 compact, sometimes araygdaloidal and decomposed forms such as are 

 known to German petrographers as dichte diabase, diabase mandelstein, 

 (amygdaloid) Icallc-diabasc, variolite, etc. ; (3) the true augite porphyrite, 

 including the normal porphyritic forms with the amorphous base, and 

 (4) the glassy variety augite vitrophyrite. 

 The following localities and varieties are represented : 



Melapkyr : Hiugham, Nantasket, and Brighton, Massachusetts, 38378, 38524, and 

 35940; Taylor's Falls, Minnesota, 26591; Berkshire Canon, Virginia Range, Ne- 

 vada, 22405; Ilfeld, in the Harz Mountains, 36474; Heiinhach, Aaweiler, Sotern 

 andErzweiler, Nahe, Prussia, 36472, 36469, 36471, and 36470; Berschweiler and 

 Niedeibrombach, Nahe, Prussia (Weiselbergite types), 70220 and 70233 ; Heister- 

 herg, Reidscheid, Oberstein, and Hopstaten, Nahe, Prussia (navite types), 70219, 

 70221 to 70223 inclusive; Braunshausen, Nahe, Prussia (olivine tholeeit type), 

 70210; Gersdorf, Saxony, 36475; Neuhaus and Lahn, Silesia, 36473 and 36470. 



Augite 2 } orphyriie : Barro de Oratorio, Rio Zubara, Proviuce de Str. Catbarina, 

 Brazil, 69915; Durham, England, 36462 ; Christiania, Norway, 70398: Upsala, 

 Sweden (uralite porphyrite), 70196; Bufaure and Fassathal, Tyrol, 36476, and 

 36461; Hof, in the Fichtelgebirge, Bavaria, 36459; Dillenburg, Baldwiustein, 

 and Langenaubach, Nassau, Germany, 36457, 36460, and 36454 ; Rubelaud, Harz 

 Mountains, Germany, 36455; Elbingerode, Harz Mountains (diabase porphyrite, 

 labradorite porphyrite), 36453; Rosenberg, Nahe, Prussia, 36458; Berneck, in 

 the Fichtelgebirge, Bavaria (spilite type), 70174; Gebweiler, Alsace, Germany 

 (labrodorite porphyrite), 36449, 36450, 36451, and 36452 ; Cottonwood Creek, Gal- 

 latin County, Montana, 38595; Cusel, Nahn, Prussia (cuselite type), 70192 and 

 70193. 



9. The basalts. 



Basalt — A very old term used by Pliny and Strabo to designate cer- 

 tain black rocks from Egypt, and which were employed in the arts in 

 early times.* 



Mineral composition. — The essential minerals are augite and plagio- 

 clase feldspar with olivine in the normal forms; accessory iron ores 

 (magnetite and ilmenite), together with apatite, are always present, 

 and more rarely a rhombic pyroxene, hornblende, black mica, quartz, 

 perowskite, hauyn and nepheline, and minerals of the spinel group. 

 Metallic iron has been found as a constituent of certain basaltic rocks 

 on Disco Island, Greenland (specimen 73164). 



Chemical composition. — The composition is quite variable. The fol- 

 lowing shows the common extremes of variation : Silica, 45 % to 55 % ; 

 alumina, 10 % to 18 % ; lime, 7 % to 14 % ; magnesia, 3 % to 10 % ; 

 oxide of iron and manganese, 9 % to 10 %; potash, 0.058 %', soda, 

 2 % to 5 % ; loss by ignition, 1 % to 5 % ; specific gravity, 2.85 to 3.10. 



Structure. — Basalts vary all the way from clear glassy to holocrys- 

 talline forms. The common type is a compact and, to the unaided eye, 



Teall. British Petrography, p. 136. 



