THE OCCURRENCE OF ALGONKIAN ROCKS IN 



VERMONT AND THE EVIDENCE FOR 



THEIR SUB-DIVISION. 



Published with permission of the Chief Geologist, United States 

 Geological Survey. 



CONTENTS. 



Geography. 



Topography. 



Geology. 



Outline of the views previously held regarding the structure and age of the 

 Green Mountains. 

 The Problem Outlined. 



Reasons f«r Refering these Rocks to the Algonkian. 

 The Upper or Mendon Series of the Algonkian. 

 The Lower or Mount Holly Series of the Algonkian. 

 Evidence of Discordance Between Mount Holly and Mendon Series. 



Lithological Differences. 



Structural Differences. 



The Conglomerate-Gneiss Horizon. 

 Review. 



GEOGRAPHY. 



The area of the Pre-Cambrian rocks forming the subject of 

 this paper ^ is quite limited in comparison with the probable extent 

 of these rocks in Vermont. Personal reconnaissance work has 

 detected them existing from the town Stratton on the south to 

 Rochester on the north, a distance of fifty miles. In only a part 

 of this area has detailed work been done, viz.; from Weston to 

 Chittenden. On the east the district is bounded by Plymouth 

 Valley ; on the west by Rutland Valley, an area of about 240 



'The work, of which this paper forms a partial result, was done under the 

 immediate supervision of Mr. Raphael Pumpelly, then in charge of the Archaean 

 Division of the United States Geological Survey, to whom my greatest thanks are due 

 for useful counsel and advice. It is not to be understood that he is necessarily in per- 

 fect accord with me in any views advanced here. 



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