PART II.

<< Previous | Table of Contents | Next >>

BASALTS.

INTERSTITIAL BASALTS.

FLUIDAL-INTERSTITIAL BASALTS.

In at least two instances (152 and 155) the above-described interstitial basalts disclose a decided tendency toward a fluidal arrangement of the plagioclase laths, and thus present a transition stage between the more typically developed interstitial basalts and what may be termed basalts with a fluidal-interstitial structure. This last-named type is well illustrated by four specimens, three of which (162, 163, and 164) were collected in different parts of the basaltic area to the southwest of Crater Lake, from which most of the interstitial basalts came, and the fourth (165) from a widely separated section, namely, from the southwest slope of Timber Crater, situated about 4 miles to the northeast of the lake.

These rocks consist of plagioclase, augite, hypersthene, and magnetite with almost no olivine. Glass is also undoubtedly present, but certainly not in large amount. The fluidal structure is due mainly to the fact that the plagioclase appears in very long and slender lath form, and to the further fact that these feldspar laths have a very marked parallelism of arrangement. The structure is indeed quite suggestive of the fluidally arranged sanidine laths in trachytes and many rhyolites. These plagioclase laths are quite sharp in outline, or, at least, this may be said to be true of the sides of the crystals. They are many times as long as wide, and have an average size of about 0.1 to 0.2 millimeter. The fluidal structure is further accentuated by the fact that the hypersthene and to a considerable extent also the augite occur in rather slender prisms, which are likewise arranged parallel to the feldspars. Hypersthene is distinctly the dominating pyroxene, being usually much more abundant than augite. It occurs only in prismatic form in crystals that measure usually about 0.02 to 0.05 millimeter in length, and about one-third that amount in width. Occasionally somewhat larger individuals may be seen, but, as a general thing, their size is very uniform. They inclose small grains and octahedral crystals of magnetite. The augite is to be seen both in grains and in minute prisms, almost identical in size and appearance with the hypersthene prisms. In fact, in white light it is almost impossible to distinguish between the two, as in color, size, shape, and inclosures they resemble each other. Even the pleochroism can hardly be used as a means for distinguishing these two pyroxenes, because of the necessarily thin sections required for the proper study of such fine-grained basalts. In polarized light, however, the distinction is not difficult. Parallelism of growth between hypersthene and augite may be observed exactly as in the above-described rocks, the augite appearing in the thin section as slender parallel strips on each side of the hypersthene.

The fluidal-interstitial structure is shown in fig. D of Pl. XIX (p. 138).

The almost complete absence of olivine in the basalts in which hypersthene is unusually abundant is a further corroboration of the frequently noticed fact that the development of olivine in a basalt is not so much dependent on the chemical composition as upon the conditions of solidification. Irrespective of the fact that the amount of olivine in these and in the more typically developed interstitial basalts appears to vary inversely with the development of hypersthene, it may be remarked that the small amount of olivine present in all these basalts falls in line with the remark of Professor Rosenbusch,^a to the effect that olivine occurs most sparingly in the basalts of a hypidiomorphic or of a doleritic type. The basalts under discussion, although hardly to be called typically doleritic, are at least more allied to that type than to any other.

^a Mikroskopische Physiographie, 3d edition, vol. II, 1896, p. 993.

In No. 163 are to be seen minute, rod-like microlites of a deep reddish-brown color and almost opaque. In size they measure on the average 0.01 millimeter long and 0.001 millimeter wide. They resemble rutile, but do not give the brilliant polarization colors characteristic of such minute crystals of that mineral. In fact, it could not be shown that they affected polarized light at all. They have not been noticed in the other basalts.

These fluidal-interstitial basalts, when compared to those already described in these pages as interstitial, are closely allied to those occurring in the northwest corner of the Crater Lake area, and more especially to Nos. 152 and 155. No. 164 shows the fluidal structure much less perfectly developed than do the other three and may be considered as intermediate between the two types. No. 166 was collected at the same place as No. 164 and is probably a locally differentiated variety. It could hardly be classed here if taken by itself. The plagioclase is much less abundant and occurs in somewhat larger slender laths, that lie in all directions but are not abundant enough to interfere with each other to any great extent. The bulk of the rock consists of a feldspathic paste thickly crowded with extremely minute pyroxene prisms and with octahedral crystals of magnetite. Both augite and hypersthene are represented, but it is impossible to determine in what proportion.

These two types of basalt are, then, very closely linked together, and the transitions between the different specimens collected are much closer than are the transitions between these and the porphyritic basalts whose description follows, and still more plainly is this true as compared with the basaltic-looking andesites of this same region. The force of this statement will better be appreciated in connection with the comparison made below with the so-called hypersthene-andesite from Franklin Hill, Plumas County, Cal. We have seen that many of the basalts and andesites of Crater Lake can with difficulty be distinguished from each other, but this can not be said of the above-described interstitial and fluidal-interstitial basalts. The more marked characteristics of these rocks for purposes of comparison with the andesites is the almost complete absence of porphyritic development. This is seen both in the scanty development of phenocrysts and in the lack of a younger generation of the mineral ingredients.

These fluidal-interstitial basalts (especially No. 162) bear a close resemblance to a rock from Franklin Hill, Plumas County, Cal., which is briefly mentioned by Mr. H. W. Turner in the Bidwell Bar folio.^a In his article on The Age and Succession of the Igneous Rocks of the Sierra Nevada^b he calls it a hypersthene andesite, and describes it as follows:

"In the area of the Downieville and Bidwell Bar atlas sheets are numerous bodies of a dense fine-grained gray lava, which usually weathers with a slaty fracture, the apparent cleavage being often vertical. The rock is composed of plagioclase, augite, a slightly pleochroic rhombic pyroxene, and grains of magnetite. The feldspar, augite, and rhombic pyroxene are in the form of minute elongated prisms or laths, and this is true of the rock at widely separated localities, and the laths of all are nearly of the same size. About half a gram of No. 661, powdered and treated with HCl by George Steiger, yielded no gelatinous silica, but nevertheless there appears to be some glass present."

^aGeologic Atlas U. S., folio 43.
^bJour. Geol., vol. III, 1895., p. 409.

Through the kindness of Mr. Diller the writer was enabled to make a study of a thin section of No. 661, referred to above. The resemblance between the rock which Turner calls a hypersthene-andesite and those just described from Crater Lake is even closer than might be inferred from the above-quoted description. The differences are rather in the size of the grain than in more important matters. One sees the same delicate, uniform prisms of rhombic pyroxene and of augite, the same slender feldspar laths, and, locally developed, in the thin section the same parallelism of structure. In the California rock the pyroxenes are somewhat larger and therefore more plainly developed. Mr. Turner also gives in the same paper a partial chemical analysis, as follows:

Partial chemical analysis of hypersthene-andesite from Franklin Hill, Plumas County, Cal.

This analysis alone certainly seems to justify Mr. Turner in calling the rock he describes an andesite, but upon close inspection it will be seen that the analysis is really that of a rock intermediate between a basalt and an andesite. As far, as the silica alone is concerned the amount is more suggestive of a hypersthene-andesite than of a basalt. But the percentage is only a little over 1 higher than that given for a basalt glass (tachylite) from Säsebühl, near Dransfeld, Hanover.^a Furthermore, Mr. Diller has published a list of chemical analyses of quartz-basalts from Cinder Cone, northeast of Lassen Peak, California,^b in which silica runs as high or higher—in one case three-quarters of 1 per cent higher—than that given for the Franklin Hill andesite. Inasmuch as Mr. Iddings has fairly demonstrated^c that the presence of primary quartz in basalts is not due to the excessive amount of silica present in the magma, it may well be claimed that the rock from Franklin Hill is not necessarily too acid for a basalt.

^aRosenbusch, Elemente der Gesteinslehre, Stuttgart, 1898, p. 309.

^bA late volcanic eruption in northern California and its peculiar lava: Bull. U. S. Geol. Survey No. 79, 1891, p. 29.

^cOrigin of primary quartz in basalt: Am. Jour. Sci., 3d series, Vol. XXXVI, 1888, p. 220.

When the lime, potash, and soda in this rock are taken into account it would be impossible to distinguish as to whether the rock were a basalt or an andesite, for it would be easy to cite analyses of both rocks in which these substances are present in almost the exact relationships that are given in the case under discussion. As to whether the Franklin Hill rock should really be called a hypersthene-basalt instead of an hypersthene-andesite the writer will not assume to say, as this might well depend on surrounding associations with which he is not sufficiently familiar. In the absence of a chemical analysis of the fluidal-interstitial basalts of Crater Lake it may perhaps be assumed that the composition does not vary greatly from that of the California rock. Even so, on account of the close resemblance to well-defined basaltic types of the same region, and on account of the sharp contrast it presents to the andesites of this same region, one may, in the opinion of the writer, be justified in calling these rocks basalts. In any case, on account of the great variability in the chemical analyses of rocks we are accustomed to class under the same family name, the structural relationships would serve better as a means for determining the rock name than would the chemical analysis alone, especially in cases where that analysis is not typical.