207 



layers in the beds of slate adjoining the granite is a plain indica- 

 tion that these are near the top of the series. The alteration of 

 the slate near the granite seems to show, however, that the 

 latter has reached its present position in part at least through 

 extravasation. 



In the relations to the granite of the narrow and tapering 

 band of slates along the South Shore Railroad and Monatoquot 

 River we have yet clearer evidence of discordant movements in 

 these two rocks ; and at some points the extravasation of the 

 granite seems beyond question. Now, since the general 

 movement has been upward on the part of the granite and 

 downward on the part of the slate, it is possible to extend the 

 explanation of the preceding paragraph and say that here, too, 

 the conglomerate is concealed beneath the slate ; but this will 

 be regarded as hazardous, since in an area so disturbed and 

 irregular as this the underlying beds might be expected to crop 

 somewhere. If, however, it is asserted that the conglomerate 

 is an overlying rock, and has been eroded away in this instance, 

 the absence of the amygdaloid still remains to be explained ; 

 for it will be noticed that, in general, wherever the conglomer- 

 ate fails to intervene between the slates and the crystallines, the 

 amygdaloid is wanting also ; and of course the latter rock 

 cannot be taken as superior to the slate. The dislocation 

 theory accounts for the absence of both the conglomerate and 

 amygdaloid, and seems fairly satisfactory ; and yet I am rather 

 more inclined toward the view, already stated {ante, p. 188), 

 that these are examples of true overlap, points where the slate, 

 during the gradual but irregular subsidence of the Boston basin, 

 has reached beyond and concealed the edges of the con- 

 glomerate. The sediments accumulating in Weymouth Fore 

 River at the present day include no coarse material, but only 

 fine mud, which, when consolidated, will form true slate. Why 

 may not similar conditions have obtained in this locality in early 

 Paleozoic time ? I presume that this river may have been then, 

 as now, a tranquil, land-locked arm of the sea, gradually silting 

 up with the finest mechanical sediment. It is interesting to 



