MUSEUM OF COMrAIiATlVE ZOOLOGY. 



23 



againsf them. The sandstones, wliicli arc of very irregular discordant 

 stratification, closely adapt thoniselves 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 tlie 

 sandstones." (/. 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, nnstratified, 

 serai-crystalline magnesiau 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, oncc-stratilied rock, which is involved in the up- 

 heaval, with its ledges bent and broken up in great confusion.'' llo 

 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. 

 *'0n the south portion of Presque Isle this dolomite is iuconformably 

 overlaid by a conglomerate and succeeding sandstone layers, which are 

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

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

 zontal.* In innnediate contact with them' they have a considerable dip, 

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

 sil)le that the strata were slightly uplifted after their deposition, but 

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

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

 erate beds at the base arc five feet thick, and contain numerous Ifrag- 

 ments of the underlying dolomitic I'ock and of their enclosed Jaspery 

 minerals." {L c, pp. 90, 91) Pie regards certain rocks at Lightdiouso 

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

 and also remarks: *' The Diorites inter stratified with the Huronian 

 schistose rocks in the environs of Marquette, and particularly at the 

 Light-house point, are of an evidently intrusive character." (L c, p. 

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

 rock can be interstratificd 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 bo pardonod for saying that, accordiii- to Messrs. rumpcUy aud llrooks 

 (soe quotations from tlieni on the Cupper district), this is proof positive tliat the Pots- 

 dam sandstones abut nnconforjnably iigamst the rrcs(jtce-idian merles, which must 

 then have formed an island in the Pot,sd;ini sea. 



t International licview, 1875, I. 754-7S0. 



