21 



that this is the true sequence of these great crystalline forma- 

 tions. This, it may be added, is the sequence deemed most 

 probable by Dr. Hunt, viz., Laurentian, Norian, Huronian. 



The Naugus Head series is certainly distinct from, and (as 

 I have already shown) probably underlies, the Huronian ; and, 

 since it bears no likeness to the Laurentian system, we are 

 brought to the conclusion, that, if it is to be correlated with any 

 series already described, that series is the Norian. In short, 

 the Naugus Head series does not resemble the Laurentian, and 

 is, stratigraphically, where we should expect to find the Norian ; 

 and these are, mainly, the considerations which led me to 

 designate this series as the "Norian," on the Centennial Geo- 

 logical Map of Massachusetts, and in my report on the same. 

 Having been informed, however, by so good an authority as 

 Dr. T. Sterry Hunt, whose opinion I had no opportunity to 

 obtain before the publication of the report above referred to, 

 that this series, chiefly on account of the supposed absence of 

 labradorite, cannot be regarded as of Norian age, I have em- 

 ployed here, and on the accompanying map, a provisional 

 designation having no chronological signification. I am con- 

 strained to believe, however, that, save in not holding labra- 

 dorite as a principal constituent, if such proves to be the fact, 

 the Naugus Head series presents a fair agreement, lithologically, 

 with the essential characters of the Norian, as the latter has 

 been described by Dr. Hunt. It is proper to state, in this con- 

 nection, that Dr. Hunt, in 1869, 1 identified, as belonging to the 

 Norian formation, a boulder found on Marblehead Neck, and 

 possibly derived from the Naugus Head areas to the northward. 

 I know by personal observation that the Naugus Head rocks are 

 scattered as erratics all over the town of Marblehead, including 

 the Neck. Prof. A. Hyatt long ago recognized the rocks about 

 the city of Salem as probably older than the petrosilex of Mar- 

 blehead Neck ; but he did not separate them from the Huronian 

 diorites of Salem, Swampscott, and Marblehead. 



1 Amer. Jour. Sci. (2) xlix., 183, 398. 



