The groundmass of this rock, were it free from plagioclase phenocrysts, would be identical with the interstitial basalts. The plagioclase laths are the dominant mineral, and appear to inclose the angular augite grains as well as the little glass present in the interstices formed by their intersection. Hypersthene, which occurs only very sparingly as a phenocryst, is more abundant in the groundmass, but as it is found here only in prismatic crystals and not in grains like the augite, it is properly not to be considered as belonging strictly to the groundmass, but rather to the older generation of crystals.
Although this rock presents an entirely different appearance from the basalt from Desert Cone, it must still be assigned to the type which has been designated as porphyritic-interstitial. It is in a way intermediate between the holocrystalline interstitial and the andesitic types.
Quite similar to this rock is No. 171, collected between Crater Peak and Sun Creek.
Still another development of the porphyritic-interstitial type of basalt is to be seen in part of the lava rocks of Red Cone. These are dark-gray, in one case red, dense, scoriaceous lavas. The phenocrysts are very inconspicuous, but may be made out under the pocket lens. They consist of yellowish to red olivine, yellow to black augite, and white plagioclase.
Under the microscope these appear to be decidedly porphyritic rocks with abundant and well-defined, though small, phenocrysts of olivine, augite, and plagioclase. The plagioclase is the most variable of the phenocrysts, as one specimen (172, collected about l mile southwest of the cone) contains almost no phenocrysts of this mineral. It occurs in rectangular, broad to narrow, lath-shaped crystals, the smaller of which graduate into the plagioclase of the groundmass. The ends of the laths are cut squarely off, as by a pinacoid, where the groundmass is distinctly glassy, but, in case the latter is more decidedly crystalline, the ends are somewhat frayed. They inclose at times a small amount of dusty-looking glass similar to that of the groundmass, but neither in shape nor in the appearance or arrangement of the inclosures do these plagioclase phenocrysts resemble those of the andesites or of the last-described basalt. Extinction angles in symmetrically cut sections, i. e., in sections perpendicular to the brachypinacoid, were measured as follows: 27°, 30°, 31°, and 34°. They are younger than both olivine and augite.
Of the pyroxenes augite is the only one of consequence. Of the four specimens studied three (174, 175, 172) contained no hypersthene and one (173) contained but one individual, which consisted of a roundish grain having a small core of hypersthene and the outer and larger part augite in parallel position. This growth of augite appears to be secondary and distinct from the more customary occurrence of augite growing around hypersthene crystals. Aside from this one case augite occurs in granular form, but even so impresses its form upon the plagioclase wherever it comes in contact. It is to be found both isolated and in nests with olivine. The color is usually greenish, but in No. 174 many of the grains have a brownish cast, or else they have greenish centers and shade into brown on the outside. It may be added that the hand specimen also discloses a few black augite phenocrysts, 2 to 3 millimeters long, that do not appear in the thin section.
Olivine is a very abundant constituent. It occurs in well-defined, sometimes in very sharply defined crystals, as well as in grains, the latter form being common where the olivine forms nests with augite and occasionally with plagioclase. In No. 174 this mineral occurs exactly as described for No. 169, one of the specimens collected on Desert Cone. It has the same clear-cut forms and has undergone the same alteration to hematite. In this case, however, the alteration has progressed still further. Not only have we the outer rim of almost opaque hematite, but the center of even the largest crystals is so thickly crowded with hematite as to leave very little clear olivine visible. (See Fig. M of Pl. XIV (p. 76.)
The groundmass of these rocks is abundant and mostly very distinct from the phenocrysts. It consists essentially of a colorless but dusty glass base that is crowded with augite microlites in the form of minute prisms and grains, also microlitic plagioclase laths and sharp magnetite octahedrons. The interstitial structure is not clearly brought out, but is produced in part by the plagioclase of the phenocrysts and in part by the plagioclase of the groundmass, there being no well-defined distinction between the two. The porphyritic structure, therefore, is much more in evidence than is the interstitial, and this rock may be considered in a sense as intermediate between the porphyritic interstitial and the more distinctly porphyritic basalts. It does not, however, bear much resemblance to that type of porphyritic basalts that are described as andesitic in these pages.
The chemical analysis of No. 173 will be found on page 161.
Two specimens, Nos. 176 and 177, collected about 3 and 4 miles, respectively, west of Red Cone, have been placed in this group of porphyritic interstitial basalts, although they are decidedly transitional between this group and the interstitial basalts proper. The greater part of the rock, as seen in thin section, constitutes a sort of holocrystalline groundmass similar to that of the more coarsely grained interstitial basalts. In this groundmass of plagioclase, augite, and magnetite occur some rather sharply defined phenocrysts of olivine, augite, and plagioclase. The two last-named phenocrysts occur also in granular form or in nests of grains, and grade off into the same minerals of the groundmass, so that no sharp line can be drawn in either case between the components of the groundmass and the phenocrysts. In the case of olivine, granular forms also occur, but these are always distinct from the groundmass in which no olivine occurs.
The olivine is slightly serpentinized, and is stained in places a deep red. It contains inclosures of magnetite. The better formed crystals of augite disclose the customary forms, namely, prism, two pinacoids, and flat terminal pyramids or dome faces. It is frequently twinned in double, triple, and multiple twins. The twinning plane is the orthopinacoid. It incloses olivine and magnetite, and, to a very slight extent, also plagioclase. The plagioclase crystals show more or less rectangular, but not very sharply cut, outlines. The period of development almost exactly coincides with that of the augite, as these two minerals often impress their form on each other. A few crystals contain minute glass inclosures, but usually they are free from inclosures, and do not bear any resemblance to the plagioclase phenocrysts of the andesites.
Hypersthene appears to be entirely wanting.
A very similar basalt, No. 178, from near the road south of Castle Creek, at the very edge of the district covered by the map, is also placed in this division.
