18 



BULLETIN OF THE 



schists, in the foldings and convolutions to which these masses of ancient 

 strata have been subjected." {L c, pp. 2G, 27.) 



In 1873 the Report of Major T. 13. Brooks on the Iron districts of 

 Michigan was published, with various accompanying documents by 

 Messrs. Julien, Wright, Houghton, Lawton, and otliers.* Mr. Brooks 

 refers the date of the discovery of the iron ore to 1844, by Mr. Wil- 

 liam A. Burt and party, but the first official documents giving an 

 account of the ore seem to have been the reports of Burt and Hubbard 

 for 1845,t referred to in the early portion of this paper. Besides the 

 sandstone, the formations of this district are divided by Mr. Brooks as 

 follows : ** The Iron-hearing Each, corresponding, it is assumed, with the 

 Huronian system of Canada, consist of a series of extensively folded beds 

 of diorite, quartzite, chloritic schists, clay, and mica slates, and graphi- 

 tic shales, among which are intercalated extensive beds of several varie- 

 ties of iron ore The Granitic Each, which so far have produced 



no useful minerals, and which are believed to be the equivalents of the 



Laurentian of Canada." (/. c, p. 06.) 



As wc shall have largely to deal with Mr. Brooks's work hereafter, wo 

 shall quote quite fully from it. '' Useful minerals which occur in beds, 

 like the iron ores of Lake Superior, will usually be overlaid and under- 

 laid by rocks having different characters, and which maintain those 

 characters for considerable distances. Next to finding the ore itself, it 

 is desirable to find the hanging or footwall rock. Whoever identifies 

 the upper quartzite in the Marquette region, or the upper marble m tho 

 Menominee region, has a sure key to the discovery of any ore that may 

 exist in the vicinity. With few exceptions, all the rocks in the region 

 wc are describing are stratified,- that is, arranged in more or less regu- 

 lar beds or layers, which are sometimes horizontal, bat usually highly 

 inclined. This stratification, or hedcUvf/, is generally indicated by a 

 difference in color of the several layers, oftentimes by a difi^crcnce m 

 tlie material itself, but occasionally the only difierence is in the texture 

 or size and arrangement of the minerals making up the rock. .... Tn 

 general, a striped rock, whether the stripes be broad or narrow, plain or 

 obscure, on fresh fracture or weathered surface, is a stratified rock." 



We would invite Mr. Brooks to inspect the volcanic rhyolites, many 

 of the felsites that are known to be eruptive, as well as many of the 

 furnace slags to be seen in the Marquette district. '^Sometimes the 

 power which produced the folds seemed greater than the rocks could 



* Geological Survey of Micliigan, I. 1 -319 ; II. 298 pp. 

 t I. 13, 14; II. 235-238. 



