BASALT DIKES. 241 



will occur near the margin of the reservoir, away from the crater. 1 More- 

 over, it is to be noted that while rnany of the exceptional modifications of 

 the magma appear to be connected with the center of eruption in Crandall 

 Basin, they are more numerous in the southern district, about the head of 

 Stinkingwater River. In describing the intrusive rocks we shall com- 

 mence with those which resemble most closely the breccias and surface 

 flows. 



Basalts. — The basalts of the dikes exhibit the same megascopical habit 

 and variability as those of the breccias. Part of them have a multitude of 

 small phenocrysts of tabular labradorite and augite, and. part carry very 

 large labradorites. They appear to be the same varieties of magma which 

 have cooled in dikes, and consequently possess a slightly different ground- 

 mass. In a large number of cases the groundmass consists of lath-shaped 

 labradorite and crystals of augite and magnetite in a small amount of 

 microlitic base. The augite is occasionally slightly pleochroic. The pheno- 

 crysts are tabular labradorite, augite, and olivine, with magnetite and stout 

 colorless apatite. In a number of dikes the groundmass contains orthoclase 

 as margins around the microlites of labradorite. One of the most pro- 

 nounced of these varieties (1325) forms a dike on the ridge south of Closed 

 Creek. Its chemical composition is given by the third analysis on page 260. 

 It is closely related to shoshonite, as pointed out in Chapter IX. Others 

 contain less orthoclase and are intermediate between shoshonite and normal 

 basalt. To this variety belong most of the dikes at the head of Miller 

 Creek and those cutting the summit of Saddle Mountain. 



In some cases the groundmass contains microlitic and globulitic glass 

 base; in others it is holocrystalline. A glassy basalt from Hunter Peak 

 contains microlitic needles of feldspar, slightly curved, and magnetite grains 

 pointed at the comers, besides augite microlites with magnetites attached. 

 In some occurrences these needles are coated with magnetite and resemble 

 thin black lines. 



It is often observed that the face of a dike along the plane of contact 

 is glassy, while the center is holocrystalline. In one instance this contact 

 facies consists of almost opaque brown globulitic glass with much iron 

 oxide in minute rods, and thin feldspar needles with long forked longitudinal 

 sections shaped like an H, the groundmass extending to near the middle of 



' L. V. Pirsson, Complementary rocks and radiating dikes: Am. Jour. Sei., 3d series, Vol. L, 1895, p. 120. 

 MON XXXII, PT II 10 



