259 



two east-west systems of the Nantasket area are represented 

 here, but there are no clearly exposed intersections fully to 

 prove it. The dikes of the south-of-east or newer system 

 evidently predoniinatc ; hut the maps render it unnecessary to 

 specify more particularly those belonging to the different 

 systems. The newest or north-south system of the Nantasket 

 area is, possibly, wholly uni'epresented. Certainly no typical 

 examples of it have been observed. No attention has been 

 2:iven to the numerous dikes of diabase traversino; the ofranitic 

 areas, further than to observe that they also show a general 

 agreement in trend with the east-west systems of Nantasket ; 

 and hence are probably mainly if not wholly of post-Cambrian 

 age. In other words, on account of the great difficulty of 

 tracing and correlating the dikes, no attempt has been made to 

 map them outside of the sedimentary areas ; and within these 

 areas it is probable that a large proportion of the dikes are 

 wholly concealed by drift, while many others are so imperfectly 

 exposed as to preclude their delineation. The maps show the 

 trend and, in most cases, the width in feet of each dike. The 

 prevailing hade, as at Nantasket, is to the north. 



Dikes of the Village Area. 



Beginning at the east, we have first the short dike, from 

 50 to 70 feet in width, in the granite between Elm Street and 

 Lafayette Avenue. This is of special interest as helping 

 to [)rove the faults which limit it on tiie east and west, and as 

 showing that in this case at least the dike probably antedates 

 the faults. The large masses of trap in the granite on and 

 near Hersey Street present no points of special interest. 

 They are, possibly, connected with each other and also with 

 the large dike in the conglomerate series which crosses Ilersey 

 Street at tlie junction with the lane. The southward extension 

 of this dike is, however, quite problematical : but the east-west 

 branching portion has been quite clearly traced out, varying in 



