76 TUE MAKQUETTE IKON-BEAlilNG DISTRICT. 



younger than tlie scliists. If these schists are Huronian, as was l)elieved 

 to 1)0 the case, then the granite is not Lanrentian. More signiticant, if 

 found correct, are the author's observations that tlie granite is intrusive also 

 in (juartzite. On the line of the Chicago and Northwestern Railway, south 

 of Tshpeming, tiie granite is mentioned as cutting a quartzite "that resembles 

 the ordinary 'Huronian' (juartzites." Southeast of Champion it is said to 

 cut a sedimentary micaceous and magnetite schist. 



If these ol)servations are correct, some of the granites of the Mar- 

 quette region are younger than the quartzites associated with the ores; but 

 apparently later observers could not anywhere find quartzite intruded by 

 granite. 



The microscopic features of the various granites mentioned by Wads- 

 worth, and of manv of their associated rocks, are described by him. At 

 Re})ulilic a fine-graincil rock, composed essentiall)' of quartz and mica, Avas 

 found in actual contact with typical granite or A'ery near the latter rock. 

 This, together with the "quartzites" of Formation XIV, at the same place, 

 is regarded as the modified edge of the granite, and, from a purely 

 petrographical standpoint, as gneiss. 



After describing the characteristics of the Potsdam sandstone, the 

 author discusses the nature of the Presc|ue Isle trap and its relations to 

 the sandstone associated with it. By microscopic examination he finds the 

 "trap" to be a ])eridotite composed of olivine, enstatite, and diallage in its 

 freshest portions, and a serpentine elsewhere. The serpentine has evidently 

 been derived from both the olivine and the diallage of the original peridotite. 

 With it is always a large (piantity of dolomite, so that it seems probable 

 that Rominger's stratified doh unite at this place is simply a very nmcli 

 decomposed portion of the peridotite. With respect to this latter rock the 

 author says (p. ()2): 



We regard this ))eridotite as an erupdve rock, younger tbau the sandstone over- 

 lying it, and agree in tliis particular with Dr. Houghton. The portion tilled with veins, 

 that was taken l)y him as a sedimentary rock belonging to the sandstone, or a mixture 

 of sandstone and trap; as a volcanic sand or ash, by Messrs. Foster and Whitney; and 

 as a dolomite, older than both trap and sandstone, by Dr. Rominger, we regard as 

 sim])ly the upper portion of the intrusive mass, modified by its contact while heated 

 with the overlying sandstone, and by the percolating waters since. 



