GEOLOGICAL EXI'LOKATIONS AKD LlTEliATUKE— 1881. 81 



Tlie evidence given in belialt' of the eruptive origin of tlie jiispilite 

 and ore are the eruptive rehitions that are shown to exist between them and 

 the associated rocks at their contacts. "Tlie jaspihte and ore are found to 

 break in various directions across tlie lamination of the associated rocks, 

 to indurate them at the line of junction, to send string-ers and tongues into 

 them, to cut the lamina' in every direction; in short, to liehave always 

 like an eruptive rock and never like a sedimentary one" (p. 470). This 

 theory assumes the sedimentary origin of the schists associated with the 

 ores and jaspilite. 



The author thinks that if the ores were originally magnetite, or if they 

 have always lieen hematite, there is no chemical diiticultv in the way of 

 believing in their eruptive origin, for magnetite is present in all eruptive 

 rocks, and hematite in many of them. 



We rest our conclusion that the jaspilite and iron ore in the Marquette district 

 are eruptive upon the tact that they possess characters which eruptive rocks exhibit, 

 especially iu relation to other rocks, and which uo sedimentary rock, proved to be 

 such, has been known to have. They otter no characters inconsistent with those that 

 known eruptive rocks have, but they do exhibit those, as said before, that no strati- 

 tied rock has, so far as our i)resent knowledge, not theory, goes. (P. 477-478.) 



The paper closes with a statement of the conditions demanded by the 

 sedimentary and the eru[)ti\'e theories. It is pointed out that the conglom- 

 erate over the ore \\-ould, according to the sedimentary theory, necessitate 

 tlie belief in a time interval between the ores and the overlying rocks, 

 whereas according to the eruptive theory this would not be required. 



RoMiNGEK, C. Marquette iron region. Geol. Surv. of Michigan, Vol. IV, 

 Part I, New York, 1881, px). xiv and 154. With map. 



In the year 1881 C Kominger, who had been ajipointed State geolo- 

 gist of Michigan to complete the survey begun under Alexander Winchell, 

 publi.shed a report based on three seasons' field work. In this time its 

 author was able to accomplish an immense amount of geoU)gical work, 

 and to accumulate a great mass of facts concerning the geology ot the 

 district studied. This report is intended as a supplement to that of Major 

 Brooks. It deals solely with the scientitic aspects of the case, while 

 MON xxviii (j 



