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sihle to be sure of its size or trend, but it is probably a dike at 

 least fifty feet wide, having a general north-south direction. 

 Obscure outcrops of several large dikes have also been 

 observed on tiie line of Beal Street, in the granite and 

 melaphyr. 



Dikes in the Ar^ea North of Lincoln Street. 



This is the area of the third special map (PL 9), embracing 

 also the islands of the harbor. We naturally notice first the 

 great masses of coarsely crystalline diabase which are such 

 conspicuous features of the map. The actual outcrops, as 

 may be seen on referring to the map, are quite insufficient for 

 the accurate delineation of these immense dikes. But, although 

 the outlines, as engraved, are largely hypothetical, they serve 

 to show their approximate extent and probable relations to the 

 enclosing strata. The long branch extending southward from 

 the western end of the broad dike which crosses Planter's Fields 

 Lane has certainly a very slender basis of facts ; but still it 

 correlates the scattering outcrops and is inconsistent with no 

 actual observation. The same is true of the western part of 

 the large dike north of Huit's Cove. If it has not exactly 

 that form, it probably has some similar form. The boundaries 

 of all the dikes are represented by continuous lines where they 

 are reasonably well determined, and broken lines where they 

 are hyj)othetical. The slate separating the two branches of 

 the large dike between the melaphyr and the north shore 

 extends at least 150 feet farther east than represented ; and it 

 is by no means certain that these two are really united on the 

 land. The clearest exposure of the contact with the stratified 

 rocks is on the north side of the southern dike, where it breaks 

 through a bed of reddish brown slate. The slate is somewhat 

 warped, the dip and strike being appreciably aflfected ; but 

 there is no marked lithological change ; and this is the general 

 fact, the slate and sandstone appearing rarely to be sensibly 



