24 



HURONIAN. 



The rocks that may be referred to this system in Massachu- 

 setts, like those of the Naugus Head series, are believed to 

 occur only in the eastern portion of the State. They cover a 

 wide area ; and, except where the Naugus Head series, Shaw- 

 mut group, and rocks of Paleozoic age face the Atlantic, they 

 form the seashore from the New Hampshire line to Plymouth. 

 This formation is bounded on the north and north-west by a line 

 running south-westerly from Salisbury through Essex and Mid- 

 dlesex Counties to Concord. Here, after giving off a long and 

 narrow deflection, which continues nearly twenty miles farther 

 to the south-west, the line bends to the south, and continues 

 through Framingham, Holliston, Medway, and Bellingham, to 

 the north-east corner of Rhode Island. On the south, it is met 

 mainly by the carboniferous rocks of Bristol and Plymouth 

 Counties. 



The Huronian area has an extreme length, measured from the 

 New Hampshire line in Salisbury to Manomet Hill in Plymouth, 

 of sixty-five miles ; and a maximum breadth of forty miles across 

 the southern end, not counting the narrow band stretching from 

 Concord to Westborough. It is almost completely divided near 

 the middle by the Primordial, and, possibly, more recent rocks, 

 which lie about the shores of Boston Harbor, extending westerly 

 to Natick, and south-westerly to Rhode Island. On the accom- 

 panying maps the Huronian series comprises the areas marked 

 as " syenite," " porphyry," and " hornblende slate," on the geo- 

 logical map of Massachusetts, prepared by Professor Edward 

 Hitchcock. 1 



The existence in Eastern Massachusetts of rocks of Huro- 

 nian age was first announced by Dr. T. Sterry Hunt, in 1871. 

 In a paper on ' « Granites and Granitic Vein-stones " 2 he speaks 

 of the " felsites"and " felsite porphyries," or " orthophyres," oc- 



1 I have marked the Huronian boundary on the map with a heavy line, for the sake 

 of greater distinctness. 



2 Chemical and Geological Essays, p. 187. 



