234 



THE POPULAR SCIENCE MONTHLY. 



Fig. 13. 



it is transformed from a peneplain to a broken country. It is 

 practically in this stage that the region now stands. It has suf- 

 fered certain slight changes by glaciation, and by small vari- 

 ations 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 con- 

 sideration of the lost 

 volcanoes. 



Fig. 13 presents a 

 partial dissection of 

 the tilted and fault- 

 ed mass, in order to 

 show the relation of 

 the peneplain, pro- 

 duced at the end of 

 the first cycle of ero- 

 sion, to the volcanoes 

 from which the la- 

 vas 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 rea- 

 son 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 chim- 

 ney 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 be- 

 fore, 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 tlie same sequence of 

 events, but in very diagrammatic style. 



The wooden working model from which several of these fig- 



