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distinct formations appear to be included here. On the north- 

 west are slates and conglomerates which have been marked as 

 Primordial ; while to the south-east, and apparently underlying 

 these, are a somewhat brecciated, though mostly fine-grained 

 or compact, felsitic rock, and a slaty rock, evidently an incipi- 

 ient amygdaloid, both of which I refer to the Shawmut group. 

 If this correlation is just, then we have here some evidence 

 that the breccia is newer than the amygdaloid. This is not 

 seen in their stratigraphic relations, which are rather obscure, 

 but in the fact that pebbles probably derived from the latter 

 rock are found in the former. I traversed this ground very 

 hastily, and am not confident that the foregoing interpretation 

 of the facts is quite correct. One and one-half miles east of 

 Charles River Village, and just north-west of where the road 

 turns south to the river, there is a small patch of breccia. It 

 contains numerous angular, usually large, irregular fragments 

 of the surrounding petrosilex, and the paste is the same mate- 

 rial more finely comminuted. 



The amygdaloid of the large area under consideration pre- 

 sents too many varieties to admit of proper description in one 

 general definition. It is rarely amygdaloidal, and nowhere 

 more perfectly so than at the well-known localities in Brighton. 

 The amygdules, here, are of pretty uniform size, the majority 

 varying from 2 mm. to 5 mm. in diameter. The minerals 

 most* commonly found in these are epidote, quartz, chlorite, 

 and calcite, though baryte, gypsum, chalcopyrite, hematite, 

 and orthoclase are also believed to occur ; several minerals, 

 especially the first two named above, are frequently concentri- 

 cally arranged in the same kernel. Besides the proper amyg- 

 dules, these minerals form many exceedingly irregular and vein- 

 like masses, traversing the rock, sometimes for a distance of 

 several feet. The Brighton amygdaloid also contains numerous 

 minute segregated masses of jasper. These are usually marked 

 by various shades of lighter and darker red, brown, yellow, 

 etc., in parallel, horizontal bands, after the manner of onyx; 

 appearing to be essentially identical in this, as in other respects, 



