50 AN OUTLINE OF THE 



of the southern coast, eastward from Somes Sound. They 

 are fully equal to the wonderful display of successive in- 

 trusions of igneous rocks along the Massachusetts coast 

 in Swampscott, Marblehead, and Beverly. 



When the observer first examines the varied features of 

 this rocky coast, his attention will be limited to the surface 

 of the rocks as now exposed. But the structural problem 

 of the Island is not simply a problem of surfaces in two 

 dimensions. It is a problem of solids in three dimensions ; 

 and the third dimension of heiglit or depth must be inferred 

 from what can be seen on the length and breadth of the 

 surface. It will be plain, when we come to consider the 

 denudation which the island has suffered, that the present 

 surface has no particular relation to the whole mass of 

 ancient country rocks and intrusive granite with which we 

 are now concerned. The present surface merely marks 

 the stage of denudation reached at this hour of geological 

 time ; the surface at earlier hours intersected the mass at 

 a greater altitude ; in later hours the intersection will be 

 carried lower down into the mass. The present surface 

 may therefore be taken, not as belonging only to the present 

 time, but as a fair sample of what would be exhibited on 

 any nearly horizontal section across the mass, not very far 

 above or below what is now seen. 



We must therefore conceive of the great granite dike 

 not only as limited by horizontal marginal lines, but as 

 enclosed by ragged walls ; and this structure must be men- 

 tally restored upward into what is now the open air, as well 

 as deeply downward into the solid earth. The original 

 walls undoubtedly termiuated in both directions ; but no 

 one shall say how far they extended at the time when 

 the granite had just made its way upwards from the deep 

 interior of the earth and frozen stiff in its new position. 

 The greater part of the intrusion is pure unmixed granite ; 

 but far up and down the walls there must have been a con- 



