306 THE JOURNAL OF GEOLOGY. 



received successive coats of other kinds of rock in after ages. The most 

 dissimilar series of formations are known to be of the same age. What is 

 happening to-day has happened in all ages. Nothing could be more unlike 

 than the deposits now forming along the various ocean shores, and in different 

 lakes and inland seas ; yet they are all of one age. Even the deposits mak- 

 ing in one and the same basin radically differ ; as, for example, along the 

 northern and southern sides of Lake Ontario ; and along the eastern and west- 

 ern sides of Lake Champlain. It would therefore seem a useless task to seek 

 for the Huronian rocks far from their native range. 



Nason' fully describes the iron ores of the porphyry region of Missouri, 

 and incidentally treats of the associated rocks. The porphyries usually show 

 evidence of bedding, but this may be that of igneous flows. The Cambrian 

 limestones and sandstones flank and rest unconformably upon the granites 

 and porphyries. The iron ore of Iron Mountain and most of the other locali- 

 ties is in veins in the massive rock, probably of water infiltrated origin ; or in a 

 residuary mantle ; or as concentrated detritus along the slopes or ravines of 

 the porphyries. In the two latter cases the ore is derived from the 

 veins. In some cases this concentration occurred before or during the 

 deposition of the Cambrian sandstones and limestones, but in other cases is 

 subsequent to the deposition of these rocks. At Pilot Knob the succession 

 from the base upward is porphyry ; conglomerate ; a slaty ripple marked strat- 

 um in contact with the ore body ; main ore body, nineteen to twenty-nine feet 

 thick ; highly ferruginous slate, one to three feet thick ; heavy beds of conglom- 

 erate with an average thickness of one hundred feet. The pebbles of the 

 conglomerate are mainly derived from the porphyries, but the regularly lami- 

 nated slate and ore have a thin bedded structure, which is such as to lead to 

 the conclusion that they are undoubtedly of sedimentary origin. 



BelP gives a general account of the Laurentian and Huronian systems, 

 and a sketch of the geology of the country extending from Lake Huron north- 

 ward to Lake Temiscaming, and from Lake Nipissing westward to the Span- 

 ish river. The Laufentian system is divided into an upper and a lower form- 

 ation. The latter consists almost entirely of fundamental gneiss, while the 

 upper Laurentian appears to consist of metamorphosed and sedimentary strata, 

 to some extent at least. 



The lower division of the Laurentian consists of red and gray gneiss, 

 usually much bent or disturbed, and having generally a rudely foliated struct- 

 ure, and a solid or massive character. The feldspar is almost entirely ortho- 



' The Iroti Ores of Missotiri, by Frank L. Nason. In Rep. Geol. Sur., Missouri, 

 for 1891-2, Vol. 2, pp. 16-69. Jefferson City, 1892. 



^ The Laiireiitiaii and Huronian Systems North of Lake Huron, accompanied by 

 Geoolgical Map. Dr. Robert Bell, Rep. of Bureau of Mines, Ontario, 189 1, pp. 63-94. 

 Toronto, 1892. 



