ANNUAL REPORT FOR 1875. 73 



ern wilderness of Wisconsin, was detailed from the field party of Prof. 

 Irving, to accompany the chief geologist during the whole trip. 



Charles E. Wright, E. M., of Marqnette, Michigan, a highly edu- 

 cated lithologist and mining engineer, who had had many years of 

 experience among the crystalline rocks of the upper peninsula of Mich- 

 igan, was engaged to meet the party at Ashland on the first of Sep- 

 tember, and to continue with it till the close of the reconnoissance. 



For the details of the work accomplished, your excellency is refer- 

 red to the reports, maps and sections of Mr. Sweet and Mr. Wright, 

 which are herewith submitted. 



Among the objects designated in advance to be accomplished by 

 the reconnoisance, were to observe the trap dykes (the Copper-bearing 

 rocks of Mr. Sweet's report) crossing the St. Croix river, and to ascer- 

 tain their direction ; to study the relation of these same dykes to the 

 Potsdam sandstone and to the Lake Superior red sandstone"; to collect 

 facts bearing upon the relative age of the Potsdam sandstone and the 

 Lake Superior sandstone; to investigate the relations of the copper- 

 bearing conglomerate to the older and the more recent formations; to 

 make a more careful and accurate geological section of Penokee 

 mountain; to determine the general direction of that range, and the 

 probable locality where it crosses the St. Croix river; to find, by long 

 journeys northward and southward on the Chippewa, Black, Wiscon- 

 sin and Wolf rivers, boundary points between the Huronian and Lau- 

 rentian formations; to discover indications of mineral deposits; and to ; 

 collect information in regard to roads, bridle paths, trails, and streams 

 navigable for canoes, in order to enable the chief geologist to give in- 

 telligent directions for conducting the survey in detail next season, 

 with economy and efficiency. 



The numerous trap dykes crossing the St. Croix river were found 

 to run in a general east-northeast and west-southwest direction, nearly 

 parallel to each other. 



There are three trap ridges between Osceola Mills and the village 

 of St. Croix Falls; whether these dykes, so called, are of the samo 

 age, is a problem to be solved by future examinations. 



Whether they are igneous in origin, or whether a portion of them 

 are of igneous origin and another portion are not, can only be deter- 

 mined by a more careful examination of their crystalline structure 

 under the microscope. The Canadian geologists have made out, on 

 the northern shore of Lake Superior, several systems of trap dykes, 

 evidently of different eruptive periods; but the geological character 

 of the region on the southern shore of the lake is so different in many 

 respects from that of the opposite shore, that no trustworthy argument 



