82 THE MAKQUETTE IKON BEAltING DISTKICT. 



Brooks's report considered the district principally from the economic stand- 

 point. The uv,\\) furnished by Kominger is extremely accurate in its 

 delimitation of the \ari<)us formations recognized by its author. It has 

 proved of incalculable value t<> the present writers in their iield work in 

 that portion of the district covered l)y it. Its topography, as well as its 

 geologv, shows evidence of the immense amount of careful labor put 

 upon it. 



In general the author regards the Marquette iron-bearing rocks as 

 Huronian and as Iving in a synclinal trough formed by the upheaval of the 

 edges of the granite basin in which they rest. By the rising of these edges 

 the inclosed sedimentary rocks were uplifted and compressed into parallel 

 folds. The uphea\ed granitic and sedimentary rocks are traversed by rock 

 belts, which represent lava streams that were intruded from below at different 

 periods after the formation of the traversed rocks. The author declines to 

 regard the granitic rocks of the region as Laurentian, since as a series they 

 do not correspond lithologicallv with the Canadian Laurentian, and since 

 the discordances described as existing between them and the Huronian rocks 

 are not discordances between the two divisions at their immediate contacts. 

 Even if discordances do exist they would prove nothing, according to the 

 author, in Ijeds so nnu-h disturbed as are those in the Marquette district 



As far as my own observations go, I have never been .able to discover any posi- 

 tive proof of an existing discordance between the granites of Marquette and the 

 adjoining Huronian beds; on the contrary, outcrops of the two kinds of rock supposed 

 to represent the coutact of the two formations exhibit every^rhere a remarkable 

 parallelism in strike and dip. and in a good many localities, where belts of granite are 

 found iuterlaminated between the Huronian schists, the conformity is perfect; but I 

 am far from believing that these conformably interstratitied bands of granite ever had 

 been formed there as regular members of the sedimentary series; I consider them as 

 iutrusive masses » * * ^vhich came to the surface after the Huronian beds were 

 already formed, and by their eruption caused not only the great dislocations of the 

 Huronian formation, but the half molten plastic granite masses induced by their cou- 

 tact with the Huronian rock beds, also their alteration into a more or less perfect 

 crystalline condition, and commingled with them so as to make it an embarrassing tiisk 

 to tind a line of demarcation between the iutrusive and the intruded rock masses. 

 (P. 6.) 



