HANDBOOK FOR THE DEPARTMENT OF GEOLOGY. 583 



homogeneous rock, with a splintery or couchoidal fracture, and show- 

 ing only porphyritic olivines in such size as to be recognizable. (See 

 specimens Nos. 34752, 35852, and 36500.) Under tbe microscope they 

 show a groundmass of small feldspar and augite microlites, with per- 

 haps a sprinkling of porphyritic forms of feldspar, augite, and olivine, 

 and a varying amount of interstitial brownish glass; the glass may be 

 wholly or in part replaced by devitrification products, as minute hairs, 

 needles, and granules. A marked flow structure is often developed, 

 the feldspars of the groundmass having flowed around the olivine be- 

 longing to the earlier period of consolidation, giving rise to an appear- 

 ance that may be compared to logs in a mill stream, the olivines repre- 

 senting small islands. (See transparencies No. 39081.) Pumiceous 

 and amygdaloidal forms are common. 



Colors. — The prevailing colors are dark, some shade of gray to per- 

 fectly black. Red and brown colors are also common. Mineralogically 

 it will be observed the basalts resemble the olivine diabases and mela- 

 phyrs, of which they may be regarded as tbe younger equivalents. 

 Indeed, in very many cases it has been found impossible to ascertain 

 from a study of the specimen alone to which of the three groups it 

 should be referred, so closely at times do they resemble one another. 



Classification and nomenclature. — In classifying, the variations in crys- 

 talline structure are the controlling factors. As, however, these char- 

 acteristics are such as may vary almost indefinitely in different portions 

 of the same flow the rule has not been rigidly adhered to here. We 

 thus have : 



(1) Dolerite, including the coarse-grained almost holo-crystallineva* 

 riety ; (2) Anamesite, including the very compact fine-grained variety, 

 the various constituents of which are not distinguishable by the unaided 

 eye; (3) Basalt proper, which includes the compact homogeneous, often 

 porphyritic variety, carrying a larger proportion of interstitial glass 

 or devitrification products than either of the above varieties, and (4) 

 Tachylite, hyalomelan or hyalobasalt which includes the vitreous or 

 glassy varieties, the mass having cooled too rapidly to allow it to assume 

 a crystalline structure. These varieties therefore bear the same rela- 

 tion to normal basalt as do the obsidians to the liparites. Other varie- 

 ties, though less common, are recognizable and characterized by the 

 presenceor absence of some predominating accessory mineral. We have 

 thus quartz, hornblende, and hypersthene basalt, etc. An olivine free va- 

 riety is also recognized. 



The basalts are among the most abundant and widespread of the 

 younger eruptive rocks. In the United States they occur mainly in the 

 regions west of the Mississippi River. They are eminently volcanic 

 rocks and occur in the form of lava streams and sheets, often ot great 

 extent, and sometimes showing a characteristic columnar structure (see 

 collections under head of structural geology). According to Richtho- 

 fen the basalts are the latest products of volcanic activity. The quartz 



