GEOLOGICAL EXPLORATIONS AND LITEEATUEE— ISG'J. 47 



be lai-o-elv eruptive. The ores l)e(lded witli cou-^-lomerates were recognized 

 as sediments. Hunt added the next important suggestion in the study of 

 the region 1)V deehn-ing the ore-bearing rocks Hurouian, and hence younger 

 than certain other portions of the Azoic, which represented the l^aurentian. 

 Kimball then made as thorough a study of the Marquette region as the 

 conditions allowed, and reached conclusions directly at variance witli those 

 of Foster and Wliitnev. Kimltall found the Azoic divisible into tlie Lau- 

 reutiau and Hurouian, to the latter of which series the ore beds belong. 

 Both the Hurouian and the Laurentian series, together with the green- 

 stones in the Huronian, were concluded to be metamorphic, while the 

 granites were thought to represent metamorphosed sediments older than 

 the rest of the Laurentian. This conclusion, of course, was directly 

 opposed to that of Foster and Whitney, who l)elieved the granites to be 

 eruptive. The ores of the Marquette district were likewise thouglit to 

 be sedimentarv exclusively. No eAddence of an eruptive origin of the ores 

 was found. 



The remainder of the publications on the district, up to the time of the 

 appearance of the abstract of this monograph, were confined largely to the 

 following problems: (1) The divisibility of the "Azoic," (2) the origin of the 

 granites and greenstones, and (3) the origin of the ores. The first jiroblem 

 could not be solved until a very detailed examination of the entire district 

 had been made. The solution of the second problem awaited the intro- 

 duction of the microscope as a working tool of the geologist. The third 

 problem became tlie principal bone of contention. 



The establishment of the ge(ilogical survey of Michigan and the 

 appointment of Maj. T. B. Brooks to investigate the iron district were 

 important steps in the solution of the three problems referred to. Since 

 the api)earance of Foster and Whitney's report in 1S51 almost nothing had 

 been added to our knowledge of the geology of the Marquette district 

 except what had been contributed by Kimball in 18G5 and by Credner in 

 18G9. The problems to be solved in- the district were so peii^lexing and 

 the physical difficulties to be overcome in solving them were so enormous 

 that it demanded the aid of the State to enable geologists to study the 

 area with anv degree of completeness. 



