. Bulletin of the Geological Society of America. Geology. CANONS AND DOMES. 69 filled with ice, as was first pointed out by Mr. Clarence King. This has been questioned; but 18 months since I found a small amount of perfectly pre- served glacial polish on the north wall. Ice, however, cannot have excavated this valley, for the moraines are of trifling extent, and facts enough have been adduced above to show that the glaciers of the Sierra have made no such canons. If the fault system studied in this paper extends to the Yo- semite, as the fissure system certainly does, the valley cannot be due
. Bulletin of the Geological Society of America. Geology. CANONS AND DOMES. 69 filled with ice, as was first pointed out by Mr. Clarence King. This has been questioned; but 18 months since I found a small amount of perfectly pre- served glacial polish on the north wall. Ice, however, cannot have excavated this valley, for the moraines are of trifling extent, and facts enough have been adduced above to show that the glaciers of the Sierra have made no such canons. If the fault system studied in this paper extends to the Yo- semite, as the fissure system certainly does, the valley cannot be due to a local subsidence. On the other hand I can see no objection to the hypothesis that the shattered zone of rock was disintegrated and eroded in preglacial times, the process being completed by a glacier which left a small moraine near the entrance and thus converted the valley into a lake. Formation of Domes.—Decomposition converts polyhedral fragments of granite into approximately spherical nodules found the world over ; and I have on previous occasions examined the cause of the process, which, shortly put, is simply that the rate of decomposition varies directly as the surface exposed per unit volume. Similarly a prism of rock projecting from a sur- face is reduced to a more or less raammillary or dome-shaped mass. The following figure shows such a mass about five feet high near Dumont meadows, at a locality where the prismatic fractures are very pronounced. In these instances examination makes it perfectly clear that the modelling is directly dependent upon the fissure systems from which decomposition has. FiGUEE 8— Weathered end of Prism. proceeded, and, as has been shown in the earlier part of this paper, the fissure systems are those to be expected in a mass which on a large scale is to be regarded as homogeneous. Thus these rounded forms are not due to any- thing like ball structure or flow structure. From such little masses as shown in figure 8 to immense masses s
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