266 BULLETIN OF THE ■ 



The main result found was that the Hanging Hills and the neighbor- 

 ing trap masses marked on Percival's Map, E. IV. 1, 2, 3, 4, 5, and 

 probably 6, and others farther nortli, are all parts of a single trap sheet 

 of overflow origin, broadly and faintly folded into a very flat synclinal, 

 perhaps a little faulted, and deeply cut by erosion around the margin. 

 It is pretty surely an overflow, because it has small inflaence on the 

 sandstone when the two are seen near together, and its upper part is 

 very amygdaloidal ; and because it makes part of the Mount Tom 

 range, whose overflow origin is well established. The former union of 

 the now separate hills is made sufficiently sure by the nearly uniform 

 height of the hills on the opposite sides of a gap (see section, fig. 41) ; 

 by the uniform slope in a single plane of the separated parts of the 

 sheet (fig. 40) ; by the conformity of this slope to the dip of the 

 adjoining sandstone, wherever seen ; and by the regular position of 

 the first anterior ridge on the slope below the great bluffs. This is 

 not apparent from Percival's description or map ; on the latter, his 

 outlines indicate only the ridges or elevations, not the areas of trap. 

 Professor Dana's reduced copy of this part of the map (c, 418) unites 

 the hills as here drawn, but he regards their trap as an intruded sheet 

 (d, 41). West Peak (E. IV. 3), the highest of the Hanging Hills, owes 

 its height over 2 and 4 to being at the southwestern end of the flat 

 synclinal ; to the same structure is due the change in the direction of 

 the bluffs at this high point from a north and south to an east and west 

 line. E. IV. 5 is higher than 4 chiefly because it is farther from the 

 axis of the synclinal ; it is noteworthy that the oblique gaps between 

 3 and 4, 4 and 5, etc., are about parallel to this synclinal axis, and may 

 very probably indicate the position of subordinate bends or breaks. 



I believe it probable and provable that Lamentation Mountain (E. 

 III. 5) is a reappearance by faulting of the Hanging Hills overflow sheet, 

 and it may extend even through E. Ill, II, and I, as is explained below 

 under faults and folds. 



F. Wallingford, Conn. (fig. 45). — The sandstone in this neighbor- 

 hood is loose and coarse, with well-marked cross-bedding ; it is cut by 

 many dikes, as was first stated by Chapin in 1835, but there are also 

 several sheets nearly conformable to the bedding. The dikes are well 

 seen on the road to Cheshire, half a mile or more southwest of the 

 Wallingford Station (New Haven and Hartford Eailroad), where they 

 break through the sandstone very irregularly : they are from five to 

 twenty feet thick, but do not affect the adjoining sandstone for more 

 than a few inches. Fig. 45 shows the ragged edge of one of them. 



