THE BASALTS 87 



talline variety; (2) anamesite, including the very compact fine- 

 grained variety, the various constituents of which are not dis- 

 tinguishable by the unaided eye; (3) lasalt proper, which in- 

 cludes the compact homogeneous, often porphyritie, variety, 

 carrying a larger proportion of interstitial glass or devitrifica- 

 tion products than either of the above varieties, and (4) tachy- 

 lite, hyalomelan, or hyalohasalt, 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 relation to normal basalt as do the obsidians to the 

 liparites. Other varieties, though less common, are recogniz- 

 able and characterized by the presence or absence of some pre- 

 dominating accessory mineral. We have thus quartz^ hornblende^ 

 and hypersthene iasalt, etc. An olivine-free variety is also 

 recognized. 



The basalts are among the most abundant and wide-spread of 

 the younger eruptive rocks. In the United States they are 

 found 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 of great extent, and sometimes show- 

 ing a characteristic columnar structure. According to Eich- 

 thofen, the basalts are the latest products of volcanic activity. 

 A quartz-bearing basalt has been described by Mr. J. S. Diller 

 as occurring at Snag Lake, near Lassens Peak, California,^ which 

 is regarded by him as a product of the latest volcanic eruption 

 within the limits of the state. 



Under the name oi-meUlite 'basalt is included a group of rocks 

 in which the mineral melilite is the characterizing constituent, 

 with accessory augite, olivine, nepheline, biotite, magnetite, 

 perowskite, and spinel. The normal structure is holocrystal- 

 line porphyritie, in which the olivine, augite, mica, or occasion- 

 ally the melilite, appear as porphyritie constituents. These are 

 rocks of very limited distribution, and at present known in 

 North America only near Montreal, Canada, Little Falls, N. T., 

 and Southern Texas. Professor Eosenbusch, in his later work, 

 separates this entirely from the basalts, and considers it in a 

 group by itself under the name of Melilite Eocks. 



^ Bull. No. 79, U. S. Geol. Survey, 1891. 



