195 



sionallj, however, we find unique evidence connecting the 

 slates of widely separated localities. Thus, the slate forming 

 East Point, Nahant, is enclosed in, and extensively cut by, 

 the coarsely crystalline pyroxenic rocks of the Naugus Head 

 series ; and near the crystallines it is usually altered in two 

 distinct ways; (1) by induration, as in the vicinity of the 

 trilobite quarry, and (2) by the development of a coarse amyg- 

 daloidal structure, which is exactly identical with that charac- 

 terizing the slate at Mill Cove. This slate, except where 

 it is vesicular, is of a uniform grayish-black color, being 

 more homogeneous than the Mill Cove rock ; but, like that, it 

 holds thin beds of limestone, aggregating, perhaps, twenty feet 

 of calcareous matter. The characters of the limestone, and its 

 mode of occurrence, are precisely the same at the two localities, 

 although thirteen miles apart ; and the evidence of synchronism 

 is much strengthened by the fact that these are the only points 

 in the Boston basin where this rock is known to occur. The 

 absence of limestone at points farther west is readily accounted 

 for by the greater shallowness, in ancient as well as modern 

 times, of the water in that direction. The often cited occur- 

 rence of fragments of Paradoxides in the drift of George's 

 Island, six miles north-north-east from Hay ward's Creek, is 

 generally and justly recognized as a strong indication that the 

 slates are of the same age on opposite sides of the basin. 



Steatigkaphy of the Boston Basin. — The idea has 

 long been prevalent, that the uncrystallines of the environs of 

 Boston form one huge synclinal through coextensive with the 

 basin in which they lie ; the slates on the north and south being 

 supposed to dip toward the centre of the basin, passing beneath 

 the great mass of conglomerate occupying that position. And 

 on this notion has been founded an argument for the more re- 

 cent age of the last-named rock. But, although there doubt- 

 less was a time, during and immediately following the deposi- 

 tion of the sediments, when the structure of the basin was 



