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THE POPULAR SCIENCE MONTHLY, 



it is easily known to be an intrusion by the dense texture of its 

 upper surface, and by the occasional brandies 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 estuarj^ was 

 filled with the detritus that was washed into it, and thus trans- 

 formed 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 



