Page:Popular Science Monthly Volume 40.djvu/241

in Fig. 7. When this first volcanic disturbance was over, the accumulation of sandstones went on again, the sands were washed in from the shores of the estuary and crept out over the back of the lava-sheet; the finer sediments settled down into the irregular crevices in the surface of the flow, even filling little half-open vesicles. A microscopic examination of specimens from these contacts of lava and overlying sandstones brings back vividly the condition of their deposition. Loose fragments of the lava, carried a little way by the waves and more or less water-worn, were Fig. 8. mixed with the sands for a few feet above the lava, but they were soon all buried. Then things went on for a long time about as before the eruption. The supply of sediments seems to have become finer after a while, for a bed of black shale is found, with numerous impressions of fossil fishes and plants, one of the few traceable fossiliferous layers of the entire formation. Then came more barren sandy shales again. It is impossible to measure the time of this quiet work in years, but after three or four hundred feet of strata had been formed, another outburst of lava (M) took place, and on a greater scale than the first. The lava-sheet formed by this eruption is three or four hundred feet thick—thick enough to have in all probability filled the shallow estuary wherever it ran, transforming it into a level lava plain, like the plain of the Shoshone River of to-day Bat the depression of the estuary trough continued; if the lava surface was at first above water level, it was soon submerged and buried in sands and mud, repeating all the significant phenomena of contact that have been mentioned above. Then came another long period of quiet, broken by a third lava outpouring (P); and after that, still more sandstones and shales, until aqueous and igneous rocks had accumulated to a thickness of perhaps two miles. At some time during this long history a sheet of lava was driven in or intruded between the sandstones near the bottom of the formation (marked I in Fig. 8);