22 BULLETIN OF THE 



deposition, if we may assume they were laid down under water, the 

 whole series, including the iron beds, have been bent, folded, and cor- 

 rugated into irregular troughs, basins, and domes, which often present 

 at the surface their upturned edges of pure ore, standing nearly 

 vertical." (/. c, p. 245.) "The trouble is to find out when a pit is 

 exhausted, — it is so common to break through a thin layer of rock, and 

 find a bed of workable ore behind it." (l. c, p. 2G2.) 



The proof advanced by Mr. Brooks to show the unconformability of 

 the so-called Huronian to the so-called Lavirentian, and the greater age 

 of the latter, is in substance this : the foliation of the latter was found 

 in two places to dip in a different direction from that of the lamination of 

 the "Huronian" schists near by {I. c, pp. 126, 156). His proof then 

 rests entirely on the hypothesis that these planes are the original bedding 

 planes of these rocks. This hypothesis again assumes that all are sedi- 

 mentary rocks, — a point still in doubt at the time of his writing, as 

 regarded one of the rocks at least. It is to be noticed that the two 

 formations were not seen in contact. He also remarks {l. c, p. 156) : 

 " The non-conformability is further proven by the fact that the Laiu-en- 

 tian generally abounds in dikes of granite and diorite, which are almost 

 entirely absent from the Huronian." Concerning certain rocks in the 

 Menominee district we find this acknowledgment : " It must be ad- 

 mitted, however, that the lithological affinities of this series of rocks 

 of the north belt are decidedly Laurentian, rather than Huronian. The 

 gneiss and granite outcrop above described, may be almost regarded as 

 a typical Laurentian rock in its appearance. If future investigations 

 prove theiu to he Laurentian, a very troublesome structural problem 

 would be presented here, as we would have Laurentian rocks conform- 

 ably overlijing beds, unmistakably Huronian. There seem to be fewer 

 difficulties in supposing that the Huronian rocks of the ]\Ienominee 

 region embrace lithological families not, so far, found rejiresented in 

 the equivalent series in the Marquette region." {I. c, p. 175.) 



Part III. of the above-mentioned work is devoted to the report of Dr. 

 C. Ptominger. We take some extracts from this : "A locality on the 

 shore, two miles south of Marquette, where the sandstones in their con- 

 tact with the Huronian Quartzites can be seen, has been previously de- 

 scribed in Foster and Whitney's report on the Lake Superior district. 

 We find here vertically erected white Quartzite beds of the Huronian 

 group projecting into the lake, which have preserved their granular sand- 

 stone structure, and are distinctly ripple-marked. They are surrounded 

 by brown sandstone and conglomerate ledges, horizontally abutting 



