MUSEUM OF COMrARATIVE ZOuLOGY. 26 



aorainst them. The sandstones, which are of very irregular discordant 

 stratification, closely adapt themselves to all inequalities of the cliffs, 

 which exhibit under the sandstone covering a rounded water-worn sur- 

 face, indicating- their long exposure before they were enveloped by the 

 sandstones." (L c, p. 90.) 



Of Presque Isle he says : " This landspur is formed by a protrusion 

 of peculiar rock-masses, differing considerably from the rock-beds of 

 the Huronian group in the vicinity. Lowest is a black, imstratified, 

 serai-crystalline magnesian 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 hy a 

 more light-colored, once-stratified rock, which is involved in the up- 

 heaval, with its ledges bent and broken up in great confusion." He 

 regards this as a dolomite. It is the same rock that Houghton consid- 

 ered to be sedimentary, and Foster and Whitney as a volcanic ash. 

 " On the south portion of Presque Isle this dolomite is inconformably 

 overlaid by a conglomerate and succeeding sandstone layei's, which are 

 identical with the sandstones of the Marquette quarries. The sand- 

 stone strata some distance off from the protrusive rocks are nearly hori- 

 zontal.* In immediate contact with them thej^ have a considerable dip, 

 corresponding to the convexity of the underlying surface. It is pos- 

 sible that the strata were slightly uplifted after their deposition, but 

 I am more inclined to explain the existing dip as au adaptation of the 

 sediments to the surface on which they were deposited. The conglom- 

 erate beds at the base are five feet thick, and contain numerous frag- 

 ments of the underlying dolomitic rock and of their enclosed Jaspery 

 minerals." (/. c, pp. 90, 92.) He regai'ds certain rocks at Light-house 

 Point and Picnic Island as intrusive diorites, giving evidence therefor, 

 and also remarks : " The Diorites interstratified with tlie Huronian 

 schistose rocks in the environs of Mai-quette, and particularly at the 

 Light-house point, are of an evidently intrusive character." {I. c, p. 

 93.) The italics are ours, as we are anxious to know how the same 

 rock can be interstratified and intrusive at the same point. 



In a paper by Prof. J. S. Newberry, on " The Iron Resources of the 

 United States," t we find the following statement : " On Lake Superior 



* May we be pardoned for saying that, according to Messrs. Fumpelly and Brooks 

 (see quotations from them on the Copper district), this is proof positive tliat tlie Pots- 

 dam sandstones abut unconformably against the Presque-islian series, which must 

 then liave formed an island in the Potsdam sea. 



t International Pieview, 1S75, I. 754-780. 



