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of the synclinal ; and the true thickness of all the slate involved 

 can scarcely exceed tAvo hundred or three hundred feet ; while 

 the volume of the conglomerate is probably not much greater. 

 Here, as elsewhere in the Boston basin, no sharp distinction 

 is possible between the brownish and grayish slates. The most 

 that can be said is , that the slate immediately overlying the con- 

 glomerate is very likely to be of brownish or purplish tints, 

 and that these colors are rare in slates not occupying this strati- 

 graphic position. 



The trend of this synclinal is much more nearly north and 

 south than represented on the map ; and, like the conglomerate 

 and purplish-brown slate on the east, it shifts to an east-west 

 course in its southern extension ; but the rocks are mostly 

 hidden in that direction, and it is highly probable, too, that the 

 syncline is changed to a faulted monocline. The gray slates 

 outcrop at two points along the road on the north side of Otis 

 Hill, with a southerly dip of 10°, or thereabouts. 



On the south side of the eastern extension of the amygdaloid 

 area, which is the cause of the swerving of the synclinal just de- 

 scribed, there is, first, a range of small pebble conglomerate and 

 sandstone, which is overlaid at one point, at least, by a grayish 

 and greenish slate precisely similar to that filling the synclinal 

 on the north ; the whole dipping south and south-south-east at 

 an ang'le of about 10°. Following this is a narrow belt of 

 amygdaloid (marking a fault) , then more conglomerate, and lastly 

 the broad band of amygdaloid lying between Otis Hill and the 

 petrosilex and granite on the south. This amygdaloidal belt is 

 broader than represented, extending nearly as far north as 

 where, on the map, the tide-water brook crosses the road to 

 Downer Landing. Proceeding from the intersection of this 

 road and Lincoln street, on a direct line towards Hewitt's Cove, 

 we have the succession represented on the map, viz., — 



Amygdaloid, the belt last mentioned. 



Conglomerate, mostly concealed and probably more or less 

 faulted. 



Slate, forming the centre of a synclinal. 



