65 Andesite Dikes

The Geology and Petrography of Crater Lake National Park, 1902




With the exception of two dikes under Llao Rock all the dikes that cut the walls surrounding the lake belong, as far as may be judged from the specimens collected, to the andesites.

Two specimens were collected from the rather large dike that runs diagonally down from The Watchman to near the lake level. These two specimens (92 and 93) may be classed with the hypocrystalline andesites, subtype B. The phenocrysts do not present any specially distinctive features, but the groundmass of the two specimens shows two extremes. No. 92 is relatively coarse grained and presents a transition to the holocrystalline type, in which, however, the allotriomorphic patches are suppressed. It is further characterized by the presence of unusually abundant plagioclase laths of a very uniform and fairly large size. No. 93, on the other hand, may be considered as transitional to subtype A in that the plagioclase laths sink to minute size. But this groundmass also contains a great many squarish or short rectangular feldspars that suggest an affinity with the dacites.

From the upper part of the dike that lies between Glacier Peak and The Watchman No. 94 was collected. In the hand specimen this presents a strikingly uniform appearance. It has a rather light-gray color, is very porous, and at a casual glance appears to be free from phenocrysts. On the contrary, phenocrysts are very abundant, as in thin section they appear to form about one-half of the entire mass. This is, in fact, a thoroughly characteristic andesite and is very strongly porphyritic, with great contrast between the phenocrysts and the groundmass. Plagioclase, hypersthene, and augite are all abundant. The pyroxenes occur both in sharp crystals and in nests of granular individuals. The feldspar occurs in two distinct types; in the larger many-faced forms having the customary glass inclusions, and in the smaller rectangular forms. From the character of the groundmass this may be considered a typical illustration of the hypocrystalline type, subtype A, characterized by the abundance of minute plagioclase microlites

The analysis of this rock is given on page 94. It indicates that this is thoroughly representative of the Crater Lake andesites.

Under the north end of Llao Rock occurs a dike of which No. 95 is a sample. It very closely resembles the rock just described. Under the microscope it appears that the phenocrysts are not quite so abundant and not so uniform in size. The groundmass contains considerable allotriomorphic feldspar, which to a certain extent hides the microlitic plagioclases. It belongs, with the above-described rock, to subtype A of the hypocrystalline andesites.

At the head of Steel Bay, a little to the east of Llao Rock, occurs a dike from which No. 96 was collected, which shows under the microscope a very pretty development of the hyalopilitic structure. Very abundant phenocrysts of the customary characters lie embedded in a groundmass consisting of a deep-brown glass that appears to impregnate a loosely felted aggregate of slender augite and plagioclase microlites. The glass base is free, or nearly free, from globulitic matter, but contains, in addition to the above-mentioned microlites, octahedral crystals and grains of magnetite.

On the east side of the lake are to be seen two dikes near Sentinel Rock. One of these (33) occurs exactly below Sentinel Rock and the other (21) about a third of a mile to the northeast. Both of these rocks have already been referred to in connection with the description of the andesites. No. 33 belongs to the subtype A of the hypocrystalline andesites and No. 21 to the hyalopilitic type. As these types have been fully described above, a repetition here is hardly necessary.

In concluding the description of the andesites in dike form, we may note that many of the various types found in the andesite flows are repeated in the dikes, while no markedly different types have been met with in the dike andesites.

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