VI PKEFACE. 



tending the scope of my thesis so as to include the whole of 

 Eastern Massachusetts, and it was accordingly finally written, 

 under the title of this paper. The thesis was accepted for 

 publication by this Society, and the Government of the Massa- 

 chusetts Institute of Technology made an appropriation to pay 

 for printing the map. 



I resolved, however, partly on the suggestion of Prof. 

 Hyatt, to test my conclusions by another year of field work, 

 and various circumstances have conspired to extend this time 

 to nearly two years. By this means my original observations 

 have been doubled, and, although the general plan of the paper 

 remains the same, it far exceeds its former limits, and many 

 of the geological boundaries on the map have been brought 

 nearer to an expression of the truth. 



In deference to the view of Dr. T. Sterry Hunt, expressed 

 to me personally, and before this Society, I have given the 

 rocks, which in the original paper were designated as the 

 Norian system, a name having no chronological signification. 

 The Huronian petrosilex and felsite are no longer regarded as 

 synchronous with the breccias ; but the latter rocks, together 

 with the amygdaloids, which appear to belong to the same 

 horizon, are set apart as a distinct formation under the name 

 of the Shawmut group. The conglomerates about Boston 

 have been found to underlie the slates, and hence are now 

 regarded as Primordial instead of Carboniferous. 



For assistance received my acknowledgments are due, first 

 of all, to Prof. Alpheus Hyatt, who generously placed at my 

 disposal his original notes, sketches, and collection, represent- 

 ing observations made during several years on the geology 

 of Essex County ; and I am also indebted to him for many 

 opportunities for investigation in the field and laboratory. 



