Of all the dikes on the caldera walls, the most conspicuous and thickest is the Devil’s Backbone. Like the dike just mentioned, it gradually changes from coarsely pilotaxitic in the center to hyalopilitic toward the margins, and is invariably rich in microlithic oligoclase. In the same direction there is a diminution in the number of cristobalite crystals lining the vesicles. Few of the Crater Lake dikes contain this mineral in greater abundance. From the analysis of a sample of the Devil’s Backbone (no. 15), the similarity of the dike rock to the andesite flows is immediately apparent.
The dike on the wall of Grotto Cove is essentially like the Devil’s Backbone. So are the two dikes on the walls below Sentinel Point and the one below Sun Notch, except that they lack both tridymite and cristobalite, and occasionally carry olivine.
Another dike, particularly interesting on account of its alteration, is the one which descends to the edge of the lake at Eagle Point. Most of this intrusion consists of dense, black andesite. In thin section, half of the rock is seen to be composed of devitrified glass rendered almost opaque by dusty ore. Almost as voluminous are large, spongy zoned phenocrysts of andesine-labradorite. The remainder is made up of hypersthene prisms (5 per cent) partly altered to ?bowlingite, less altered crystals of augite (3 per cent), and rare grains of olivine. Contrasting strongly with this dark andesite is a pale sage-green “propylitic” type which is largely confined to the margins of the intrusion. Normally, as we have seen, the edges of dikes are finer-grained than the interior, but here the reverse is true. Moreover, the pyroxene crystals, and especially those of hypersthene, are more thoroughly altered to serpentine than in the central part of the dike. Calcite is sprinkled throughout, particularly along cracks in the porphyritic feldspar. Finally, the joint faces are lined locally with opal and radiating prisms of quartz. Since in most other dikes it is the central part which is most altered, it may be supposed that the changes visible in the Eagle Point intrusion are the result, not of the concentration of residual solutions during solidification, but of later solfataric action which has similarly affected the neighboring lavas.
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