STRUCTURE AND STRATIGRAPHY. 27 



more numerous, but owing to the fact that the strata stand on edg-e, due to 

 the closer folding of the rock series here, a more accurate estimate of their 

 thickness can be made. 



According to Smyth, this limestone formation, in the southeastern end 

 of the ellipse, at its uj^per horizon becomes mixed with slates, and these 

 increase in quantity until the formation passes above into a slate formation, 

 called the Mansfield slate.^ This slate formation is found overlying the 

 limestone to the west of the central ellipse likewise, but as few outcrops 

 have been found, it is not positively known to exist as a continuous zone 

 encircling the northwestern end. In a direct line with its probable continua- 

 tion to the north, a graywacke was found at one place, sec. 19, T. 46, R. 32. 

 This single outcrop is insufficient evidence to warrant the introduction of a 

 graywacke formation as the northern equivalent of a part of the Mansfield 

 slates, and it is probably but a phase of that formation. The only mine of 

 this district producing Bessemer ore is in a deposit in the Mansfield slate. 



The close of the Mansfield Slate time was marked by the extrusion of 

 a great series of volcanics, which constitute the next formation in the 

 succession. This volcanic formation has its best and most typical develop- 

 ment west of the western Archean ellipse. Because the Hemlock River 

 and its tributaries have exposed good sections in the volcanics, and because 

 this river drains a great portion of the volcanic area, the name ' ' Hemlock 

 formation " is applied to the volcanics. The dip of the flows and of the tuff 

 beds wherever observed is about 75° west. The maximum breadth is about 

 5 miles. Deducting 15° for initial dip, this would give the enormous maxi- 

 mum thickness of 23,000 feet to the volcanics, upon the supposition that no 

 minor folds occur. 



These volcanic rocks have associated with them rocks of unquestionably 

 sedimentary origin, as is shown by their well-bedded condition and the 

 rounding of the fragments. The subaqueous rocks are, however, composed 

 of little-altered volcanic materials, and evidently point to oscillations of the 

 crust during the time of volcanic activity — such oscillations as have long 

 been known to be common in volcanic regions. 



Following the volcanics, and overl5^ing them, probabl}" unconformably, 

 comes a series of sedimentary rocks, believed to belong to the Ujjper 

 Huronian. These comprise chloritic, ferruginous,' and carbonaceous slates, 



'See Part II, Chapter IV, Sec. IV. 



