Page:Popular Science Monthly Volume 40.djvu/246

234 it is transformed from a peneplain to a broken country. It is practically in this stage that the region now stands. It has suffered certain slight changes by glaciation, and by small variations of level; but its main features are explained in accordance with the scheme thus presented; and from this general sketch we may return to the more especial consideration of the lost volcanoes.

Fig. 13 presents a partial dissection of the tilted and faulted mass, in order to show the relation of the peneplain, produced at the end of the first cycle of erosion, to the volcanoes from which the lavas were poured out. The near corner block is stripped down to the present stage of topographic form; the second represents the peneplain stage; the other three retain their constructional form. It is here made apparent that by reason of the tilting, the volcanic cones were raised above the old base-level of erosion, and were hence doomed to destruction in the process of base-leveling. The further edges of their flows remain; the stump of the long chimney up through which their lavas rose to the surface is still discoverable, but the cones, where the chimney rose to the surface and gave forth the flows, are lost. Fig. 11, which represents the completed peneplain, has no trace of them, although the edges of the flows and the stump of the chimney can be identified. Fig. 13. illustrating the present form of the surface in a general way, shows no volcanoes, but it shows the edges of the flows and the stump of the chimney better than before, because they, being hard rocks, have held up their edges, while the surrounding weaker sandstones and shales have wasted away. Thus the Blue Hills have been developed; not by lifting up their heavy summits above the surrounding surface, but by holding hard to the form that they had at the end of the previous cycle, while the surrounding rocks have lost it. Denudation has not yet progressed deep enough to reveal the connection that very likely exists between the chimney and the lower intrusive sheet; this is still buried. Fig. 14 tells the same sequence of events, but in very diagrammatic style.

The wooden working model from which several of these