86 EOCKS FORMED THEOUGH IGNEOUS AGENCIES 



kalk-diahase, variolite, etc.; (3) the true aiigite porpkyrite^ in- 

 cluding the normal porphyritic forms with the amorphous base, 

 and (4) the glassy variety augite vUrophyriie. 



(4) THE BASALTS 



Basalt, a very old term used by Pliny and Strabo to designate 

 certain black rocks from Egypt which were employed in the arts 

 in early times.^ 



Mineral Composition. — The essential minerals are augite and 

 plagioclase feldspar with olivine in the normal forms ; accessory 

 magnetite and ilmenite, together with apatite, are always pres- 

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

 quartz, perowskite, hauyne and nepheline, and minerals of the 

 spinel group. Metallic iron has been found as a constituent of 

 certain basaltic rocks of Greenland. 



Chemical Composition. — The composition is quite variable, 

 as shown by analyses in columns V and VI on p. 83. 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 16% ; potash, 

 0.058% to 1.50% ; soda, 2% to 5% ; loss by ignition, 1% to 5% ; 

 specific gravity, 2.85 to 3.10. 



Structure. — Basalts vary from clear glassy to holocrystalline 

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

 homogeneous rock, with a splintery or conchoidal fracture, show- 

 ing only porphyritic olivines in sucli size as to be recognizable. 

 Under the microscope they show a ground-mass of small feldspar 

 and augite microlites, with perhaps a sprinkling of porphyritic 

 fornxs of feldspar, augite, and olivine, and a varying amount of 

 interstitial brownish glass. Pumiceous and amygdaloidal forms 

 are common. 



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

 to perfectly black. Red and brown colors are also common. 



Classification and Nomenclature. — In classifying, the varia- 

 tions in crystalline structure are the controlling factors. As, 

 however, these characteristics 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 holocrys- 



^Teall, Britisli Petrography, p. 136. 



