64 Uncertain Types

The Geology and Petrography of Crater Lake National Park, 1902

 PART II.

HYPERSTHENE-ANDESITES.

CLASSIFICATION OF ANDESITES.

UNCERTAIN TYPES.

With the exception of those of Wizard Island, the andesite lavas of Crater Lake do not appear to be distributed in accordance with the types herein described, but the various types occur over all the area, among both the older and the younger andesite flows. In the case of Wizard Island a partial exception may be noted in that the more thoroughly crystalline types are almost wanting. The andesites of Wizard Island are unusually black and basaltic-looking, and contain much brown glass. Occasionally they are stained a deep brownish red by iron oxide (87, 89). Some of the more glassy black portions seem to have cracked in the process of cooling, the cracked surfaces being left with a decided gloss. This was particularly noted on a specimen collected by the writer from a large block in the crater on the summit of the island.

Of the thirteen rocks from Wizard Island studied in thin section, five are of the hyalopilitic type; six of the hypocrystalline type, subtype A; two of the same type, subtype B; while only one, and this a somewhat doubtful one, is of the holocrystalline type.

Among the andesites collected by Mr. J. S. Diller on Wizard Island are ten Nos. 81 to 90, inclusive, the exact location of which is not given. Of these No. 83 has no corresponding thin section. The others do not vary greatly from the other specimens from this island enumerated above. Nos. 82, 86 and 88 are placed in the hypocrystalline type, subtype A; or possibly 82 might equally well be put in the hyalopilitic type. No. 84 is a hyalopilitic andesite. Nos. 88, 89, and 90 are so strongly impregnated with red hematite powder as not to be easily classified. They are probably of the hypocrystalline type, subtype A. No. 85 is a holocrystalline andesite with basaltic appearance. It contains small augite prisms, too large to be called microlites, and also considerable allotriomorphic feldspar in the groundmass but no poikilitic patches such as occur in most of the holocrystalline andesites.

At the head of Steel Bay, just to the east of Llao Rock, occurs an andesitic rock close to the dike from which No. 96 (page 93) was collected. This andesite is represented by No. 91, and is a nearly holocrystalline rock with very abundant phenocrysts and with a groundmass that resembles that of some of the basalts, notably Nos. 176 and 177 of the porphyritic interstitial basalts. The main features that distinguish this rock from the basalts are the absence of olivine and the very marked porphyritic development. This andesite does not closely resemble any other andesite from Crater Lake, at least not close enough to justify putting it in any of the above described types. It more closely resembles the basalts than any other rock collected on or within the crater rim.

 << previousnext >>