This type of basalts may be
said to contain well-defined and usually abundant phenocrysts, always of olivine
and generally also of plagioclase, hypersthene; and augite, in a groundmass that
bears a more or less close resemblance to the interstitial basalts. As a rule,
however, the groundmass is inclined to contain more glass and the minerals of
the second generation have more sharply developed forms than is the case with
the corresponding minerals that compose the interstitial basalts.
The lava of Desert Cone,
immediately north of Red Cone, may be taken as the best example of this type of
structure. Red Cone also is composed of basaltic lavas that belong to this type,
but not entire so, as will appear later.
The lava of Desert Cone is
represented by three specimens—No. 167 from the north side, near the summit, and
Nos. 168 and 169 collected close together about 1 mile farther south on the
southern slope. All three are very much alike. In the specimen from the north
side olivine is the only phenocryst; in the two from the southern slope, in
addition to olivine, plagioclase, in a rather subordinate role, occurs as a
The olivine in these specimens
is to be seen in very sharp idiomorphic crystals and also in crystal fragments.
They are particularly well adapted to a study of the crystallographic forms and
optical properties. At first glance some of the olivine crystals, especially the
fragments, resemble augite somewhat, the color being a light yellowish green.
Irrespective of the crystal form, however, one can usually distinguish both the
crystals and the fragments by their unusually rough appearing surface and by
their high interference colors, also by the absence of well-defined cleavages.
They usually appear in rectangular or hexagonal shapes. The dominant forms are
the brachydome (021) and a prism. In sections cut parallel to the macropinacoid
this combination gives hexagonal outlines, the angle formed by the trace of the
brachydome being not far from 80°. Such a section also gives a positive
bisectrix, with large optical angle (168).
In No. 169 the olivine
phenocrysts have very sharply developed forms and show an extensive alteration
to iron oxides. This alteration product occurs both as a broad rim and as finely
granulated masses scattered throughout the crystal. In the very smallest
crystals the olivine has been almost completely replaced; in the larger ones the
unaltered olivine rarely composes more than one-half of the entire bulk and
generally much less than one-half. This iron oxide alteration product consists
sometimes of perfectly black, opaque material, which is presumably magnetite; in
other cases the grains are slightly transparent and are of a deep red color,
suggestive of hematite. Still more commonly both magnetite and hematite abound.
In spite of the sharpness of the crystal forms the olivine phenocrysts are
frequently penetrated by plagioclase laths, thus indicating a very basic
feldspar. (See fig. L of Pl. XIV, p. 76.) These penetrating feldspar
laths do not occur in the center, but only at the margin of the crystals;
occasionally, also, one may see an olivine crystal impressing its form on a
plagioclase phenocryst. This, however, is by no means as common as the other
case. It would seem from this that the feldspars had commenced to crystalize out
before the olivines had ceased to grow. Possibly they may have begun
simultaneously with the olivines.
The only other phenocryst in
this basalt is plagioclase, and even this is absent from one of the three
specimens (167). It is by no means conspicuous nor very markedly different from
the smaller feldspathic laths that make up the greater part of the groundmass.
The shape is rectangular or long rectangular. The largest extinction angles
noticed on sections cut perpendicular to the albitic twinning plane was 30° in
No. 168 and 29° in No. 169. This does not indicate so extremely basic a feldspar
as its relationships to olivine would suggest. Probably if the plagioclase
phenocrysts were more numerous sections could be found showing larger extinction
The groundmass appears to be
nearly holocrystalline. It is composed of well-developed plagioclase laths of
hardly more than microlitic proportions, augite in minute grains, or in both
prisms and grains, and abundant magnetite in mostly very minute octahedrons and
grains. Hematite powder is also abundantly developed in the groundmass of No.
169 as well as in the olivine phenocrysts. In addition to the above-named
minerals hypersthene has been recognized in No. 168. It is entirely confined to
the groundmass and occurs in but one generation. It forms very small prisms that
measure 0.06 millimeter long by 0.01 millimeter wide and that very closely
resemble similar prisms of augite. They are too small to show any pleochroism
and are to be recognized only by their parallel extinctions and lower
interference colors. In fact, it requires some little familiarity with these
rocks before the distinction between the pyroxenes can be made.
In the southwest corner of the
area covered by the Crater Lake map, and about half a mile south of the road,
was collected a specimen of basalt, No. 170, that is closely connected with the
interstitial basalts. In a groundmass composed of plagioclase laths, augite,
hypersthene, and a little dust-laden glass occur perfectly developed
comparatively large plagioclase phenocrysts that exactly resemble the andesitic
plagioclases; also an occasional hypersthene, augite, and blood-red olivine
crystal. The plagioclase is in broad crystals that show rectangular and
six-sided outlines and, as in the andesites, zonal structure. The interior is
crowded with glass inclosures, while the margin is free from the same, or else
both the interior and the margin are clear, while the inclosures are confined to
a narrow intermediate zone. One of these phenocrysts, cut perpendicular to the
brachypinacoid, gave an extinction angle of 34°.
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
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
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
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.