it is easily known to be an intrusion by the dense texture of its upper surface, and by the occasional branches that rise from it into the overlying beds, and by various other features in which it differs distinctly from the overflow sheets or extrusions. But it need not be further considered now.
In order to exhibit these relations of the igneous rocks to the stratified deposits in a clearer manner, the model is constructed so as to open on a diagonal section ( as in Fig. 9), and disclose the
Fig. 9.
pipe or chimney up through which the lavas rose from their deep source. The volcanic cones, presumably formed at the surface where the chimney opened at the three times of eruption, are here placed in their proper positions in the series of stratified deposits; but even the topmost cone is supposed to have been entirely buried by gradual submergence and by the accumulation of sands and muds upon it. The intrusive sheet is shown near the bottom of the stratified series. The whole series may then be named as follows. First, a moderate thickness of bottom sandstones, often conglomeratic; then, the intrusive sheet; next, the great series of lower sandstones and shales, also sometimes conglomeratic; then, the three extrusive sheets, with their intervening sandstones and shales. The first of the extrusions will be called the anterior sheet, the middle one is the main sheet, the third is the postorior (for reasons that will appear more clearly further on), and they are separated by the anterior and posterior shales respectively. On the top of all come the upper sandstones and shales. The whole series is probably two miles thick, as already stated.
We may imagine in a general way that in time the estuary was filled with the detritus that was washed into it, and thus transformed into a lowland plain, like that of the Po, between the Alps and the Apennines; or like the plain of California, between the Sierra Nevada and the coast range. If it was not ultimately filled
