1884.] 25 [Crosby. 



is part, although, through excessive disturbance, possibly now 

 an isolated part, of an east-west belt of slate extending from 

 Boston across Brookline and Newton to the Charles River at 

 Newton Upper Falls. This belt, which appears to have a breadth 

 of a mile in Brookline, contracts rapidly toward the west; and in 

 this contracted portion it is well exposed ; in and around Chest- 

 nut Hill Resevoir, where it is scarcely less than one-fourth of a 

 mile wide ; on Beacon Street, Newton Centre, where it can 

 hardly be more than 250 feet wide ; and in the vicinity of New- 

 ton Upper Falls, where the width is probably not less than 600 

 feet. The dip of the slate is northerly at all points and very 

 variable, ranging from 20° to at least 70°, although usually 

 between 30° and 40°. Lithologically, this slate shows but little 

 variation, being gray, distinctly bedded, with usually a very per- 

 fect cleavage parallel with the bedding and frequent contortions. 



The slate belt is bordered on either side by a broad belt of 

 conglomerate. The conglomerate on the south side of the slate 

 is three-fourths of a mile to a mile wide, and that on the north is 

 about two-thirds as wide. But we have no reason to suppose 

 that the actual thickness of the conglomerate in the two belts 

 differs materially ; for the southern belt is characterized by gen- 

 tler dips than the northern, the beds on the southern border of 

 the southern belt being sometimes nearly horizontal, while in the 

 north belt the dip is sometimes nearly vertical. At all points the 

 dip is northerly, and averages about the same as for the slate. I 

 think it is impossible to establish any lithological distinctions 

 between these two belts of conglomerate, but they appear, rather, 

 like different parts of the same formation. Furthermore, it is 

 almost certain that at the western end they are continuous around 

 the slate, as shown on the map accompanying my "Contri- 

 butions." 



Outside of the two conglomerate belts are two broad belts con- 

 sisting chiefly of basic eruptive rocks which I have mapped as 

 amygdaloid. It is probable that these are in part older than the 

 conglomerate. But, whatever their age, it is certain that the 

 southern amygdaloid belt marks the position of an important anti- 

 clinal axis ; and it is probable that the northern belt does also. 



To my mind, these facts suggest very forcibly that the two 

 belts of conglomerate, with the belt of slate between them, are 

 involved in a closed synclinal fold having its axial plane inclined 



