CHAPTER IV. 



THE LOWER HURONIAN SERIES. 



This series is represented in the Crystal Falls district by the following 

 formations, given in order from the Dase upward: The Randville dolomite, 

 the Mansfield slate, and the Hemlock formation. At the' beginning of the 

 deposition of the Lower Huronian series the entire district was covered by 

 the pre-Cambrian sea, with the possible exception of a small island in the 

 Archean area. 



SECTION I.— THE RANDVILLE DOLOMITE. 



The best exposures of this dolomite are found near the center of the 

 district east of the western ellipse and in the extreme southeastern part of 

 the district in the Felch Mountain range. Both areas are described bj- 

 Smyth, to whom we owe the name, and the reader is refen-ed to his descrip- 

 tion on p. 406 and p. 431 for the detail characterization of the formation. 

 It will suffice for our purpose to state that it is a medium-grained crystallme 

 dolomite. 



The few outcrops which I shall mention are important as showing the 

 relations of the formation to the underlying rock, but are, petrographically 

 considered, rather exceptional phases of the formation. Hence my descrip- 

 tion will be brief 



DISTRIBUTION, EXPOSURES, AND TOPOGRAPHY. 



The area in which the Randville dolomite immediately underlies the 

 drift is a continuous zone adjacent to and surrounding the Archean core. 

 The belt varies slightly in width along the sides of the ellipse. At the 

 ends it is two or three times the width at the sides. This is due to the 

 lower dip of the beds at the ends. Exposures are found m the area studied 

 by me only on the northeast and southwest flanks of the granite core. 



The west branch of the Fence River follows the limestone area for a 

 short distance in the northeastern part of the district, skirting the Archean 



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