60 THE MAP.QUETTE IRON-liEAIJIXd DISTRICT. 



The unconformity mentioned by Foster and Wliituey as occurring' 

 near the Carp River is described by Rominger in these words (p. 90): 



We fiiul here vertically erected white quartzite beds of the Huroiiiau group, 

 l^rojectiiig iuto the lake, which have preserved their granular sandstone structuie 

 and are distinctly ripple-marked. They are surrounded by brown sandstone and. 

 conglomerate ledges, horizontally abutting again.st them. The sandstones, which 

 are of very irregular discordant stratification, closely adapt themselves to all inequali- 

 ties of the cliffs, which exhibit under the sandstone covering a ronnded, water-worn 

 surface, indicating their long exposure before they were enveloped by the sandstones. 



lie describes Presque Isle as formed by — 



a protrusion of peculiar rock masses, difl'ering considerably from the rock beds of the 

 Huronian group in the vicinity. Lowest is a black, unstratifled, semicrystalline mag- 

 nesian rock, resembling a half-decomposed basalt or a highly ferruginous serpentine. 

 It forms considerable cliffs at the north end of the spur; more to the south we find it 

 overlaid by a more light-colored, once stratified rock, which is involved in the upheaval, 

 with its ledges bent and broken up in great confusion. * * * The principal rock 

 mass, which is found in all forms, from compact crystalline to an absorbent, earthy 

 condition, is chemically a dolomite. * * * 



On the south jjortion of Presque Isle this dolomite is uuconformably overlaid 

 by a conglomerate and succeeding sandstone layers, which are identical with the 

 sandstones of the Marquette quarries. The sandstone strata some distance off from 

 the protrusive rocks [the "dolomites"] are nearly horizontal. In immediate contact 

 with them they have a considerable dip, corresponding to the convexity of the 

 underlying surface. It is possible that the strata were slightly uplifted after their 

 deposition, but I am more inclined to explain the existing dip as an adaptation of 

 the sediments to the surface on which they were deposited. The conglomerate beds 

 at the base are 5 feet thick and contain numerous fragments of the underlying 

 dolomitic rocks and of their inclosed jaspery minerals. (P. 92.) 



On Ligltt-House Point the author noted that the bands of diorite, 

 whicli had been so frequently mentioned by earlier authors as interstratified 

 with the Huronian schists, "are connected among themselves by transverse 

 bands cutting across the strata of tlie schists," and therefore the diorites 

 are believed to be intrusive (p. 03). 



Rominger thus makes the Prescpie Isle dolomitic rock older than the 

 sandstones associated with it, and the green schists of Light-House Point 

 he regards as intrusive. 



