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SUPPLEMENTAEY NOTE. 



Recent observations have convinced me that the isolated 

 patch of slate lying upon and cut by the Blue Hill granite, near 

 the common boundary of Quincy and Milton, is much larger 

 than represented on the map, and that it belongs to the Pri- 

 mordial series instead of the Shawmut group. The new 

 evidence consists of several ledges of a distinctly stratified 

 gray argillite on Randolph turnpike, in Milton, about three- 

 fourths of a mile north of the Quincy line. The dip is verti- 

 cal and the strike E.N.E. ; and the character of the rock is, 

 in general, precisely the same as in the Paradoxides quarry in 

 Braintree, but some layers are more pyritiferous. So far as 

 exposed, the slate is not traversed by exotics, though a large 

 mass of coarse diorite or diabase appears to come between it 

 and the granite on the south. As compared with these ledges 

 of slate, the small area of this rock, some three-fourths of a 

 mile farther east, shows much greater disturbance, and is in 

 every way more ancient-looking ; and yet I judge that the 

 two patches are, or have been, connected* They are both 

 included in a broad, smooth valley, which divides the Blue 

 Hills longitudinally, extending from the west boundary of 

 Quincy, in a W.S.W. direction. Outcrops are very scarce in 

 this well-marked intervale ; but I think there are good reasons 

 for believing that a considerable area of Primordial slate is 

 probably concealed here. 



