238 



a nearly north dip at an angle of about fifteen degrees. Im- 

 mediately above these and without any discordance comes the 

 first of the conglomerate beds, which consists of a bed about 

 ten feet thick of pebbles mingled with slates. This is sur- 

 mounted by about thirty feet [traverse measure?"! of slates 

 having a most perfect cleavage in the plane of stratification. It 

 is not difficult to split a sheet a foot square having a thickness 

 of not over one-twentieth of an inch. Immediately above this 

 slate the cono-lomerate comes in again and continues with its 

 northern dip for a horizontal distance of over a mile." 



This slate belt, still narrowing, crops next on the north 

 side of Beacon Street, just east of Newton Centre, and near the 

 western end of the long tunnel on the new or Sudbury River 

 aqueduct. At this point we have, from the New York and 

 New England Railroad (Woonsocket Division) northwards, 

 the section represented in PI. 5, fig. 1. It begins with the 

 northern border of the great mass of conglomerate which 

 spreads south of the railroad, with rapidly diminishing dip, for 

 nearly a mile. This is cut off by amygdaloid, which forms a 

 band less than fifty feet wide, running parallel with the strike 

 of the conglomerate and the master-joints, which are well devel- 

 oped here, and traceable in that direction for about one-fourth 

 of a mile. This is one of the many isolated masses of amygda- 

 loid cutting through this belt of conglomerate. The rock is 

 the usual chloritic variety, and only slightly amygdaloidal. It 

 is bounded on the north by about twenty feet of brownish-red 

 slate, ripple-marked, and including sandy beds with layers of 

 pebbles. The contact of the slate and amygdaloid can be 

 distinctly observed at several points, and it places beyond 

 question the exotic nature of the latter rock. Above the red 

 slate, which is merely a part of the conglomerate, there are at 

 least five hundred feet, traverse measure, of small-pebble con- 

 glomerate, with limited sandy layers, reaching to the south 

 side of Beacon Street. There are no exposures in the street, 

 but it is probably underlaid here by slate ; and on the north 

 side is the well-known ledge of this rock. The bedding of the 



