62 BULLETIN : MUSEUM OF COMPARATIVE ZOOLOGY. 



the Boston area. The conglomerate dips beneath the slates to the north 

 and to the south and pitches eastward beneath the slates which underlie 

 Boston Harbor. Outcrops in many parts of the area are abundant, and 

 all accord with the idea that this belt is a broad, flat-topped anticline. 



The structure of the northern conglomerate belt is not so clearly 

 shown. On the southern side of the melaphyr in this area, the sedi- 

 ments dip toward it at prevailingly low angles. On Grant Avenue, New- 

 ton Centre, north of Commonwealth Avenue, the conglomerate beds are 

 practically vertical, but in no other instance has a dip of more than 25° 

 been seen. The average dip is probably below 20°. North of the mela- 

 phyr the dip is still toward the north, but usually at higher angles, 

 reachiug 60° to 65° on the railroad west of Newtonville. The attitude 

 of these rocks does not suggest the anticlinal relation. It is much 

 simpler to regard the structure as monoclinal, the dips becoming steeper 

 toward the north. 



This view is strengthened by a study of the relation which the con- 

 glomerate bears to the slate belt to the south. If the two conglomerate 

 areas were anticlinal, the intervening slate would, of necessity, be syn- 

 clinal. The conglomerate to the south appears to pass with perfect 

 conformity beneath the slate. It is reasonably clear that the slate ac- 

 tually overlies this conglomerate. But the conglomerate of the northern 

 area invariably dips away from the slate, and is everywhere discordant 

 with it. The conglomerate dips northward at angles of from 10° to 25°. 

 The slate dips in the same direction, but at angles varying from 5° to 

 90°. It is clear that there is some sort of break along this line of con- 

 tact. Fortunately several exposures yield positive evidence of the 

 nature of this break. 



On the northeast bank of Chestnut Hill Reservoir (Plate 2, Loc. 19) 

 the slate is seen interbedded with coarser sediments and clearly over- 

 thrust by a massive bed of conglomerate. The slate dips north at 

 about 80°, while the conglomerate above has a dip in the same direc- 

 tion of 30°. The occurrence of slate associated with bands of coarser 

 sediment is suggestive of the transition zone between the slate and the 

 conglomerate. The same conditions may be seen on the railroad near 

 the Pumping Station and also on the track a half-mile to the eastward. 

 It is quite possible that the thrust produced at this point a syncline 

 which finally yielded to the strain. The attitude of the rocks at the 

 thrust is quite in accord with this view (Fig. 1). The effect of the 

 thrust is observable in the slate on the neck of land separating the two 

 parts of the reservoir. The slate at this point is excessively crumpled. 



