PRE-KEWEENAWAN FORMATIONS 697 



Extrusive and intrusive greenstones. — Abundant exposures of 

 greenstone occur throughout four square miles in T. 41 N., Rs. 43 

 and 44 W., and in a few other scattered small areas. These are 

 mainly of extrusive types, but some coarsely crystalline intrusive 

 equivalents are associated with them. Later dikes of diabase, 

 probably of Keweenawan age, intrude the greenstone. The intru- 

 sives are mainly volcanic agglomerate and breccia, but in associa- 

 tion with them are amygdaloidal lavas, tuffs, and some slate and 

 conglomerate. An interesting feature of the lavas is a clearly 

 indicated genetic relation to inclosed bands and irregular masses of 

 red jasper. Basic flows with well-developed amygdaloidal tops 

 are interbedded with the Ironwood formation from as far west as 

 the Pahns mine eastward to near Wakefield but are thus far not 

 known to occur on the east end of the range. 



The volcanics are closely associated with the Ironwood forma- 

 tion, in part interbedded in its upper horizons but in greater part 

 superimposed. That they are unconformably overlain by the 

 Copps group is an inference based on general field relations and the 

 inclusion of pebbles of the volcanics in the Copps conglomerate 

 rather than direct observation of contacts. Wherever the Copps 

 group is in juxtaposition to the volcanic greenstones it invariably 

 dips away from them. 



The evidence indicates that volcanism was active on sea and 

 land from middle Ironwood time, continuing perhaps for an 

 indefinite period after the close of sedimentation. 



The Presque Isle granite. — We propose the name Presque Isle 

 granite for the rocks described by Van Hise and Irving as consti- 

 tuting the "Eastern Laurentian Granite Area.""" 



as observations have gone it may be said that this conglomerate bears no structural 

 discordance to the underlying member of the Ironwood formation. 



Mr. Fred Wolff has recently described the occurrence of a conglomerate in 

 middle horizons of the Biwabik formation on the Mesabi iron range (Engineering and 

 Mining Journal, C, No. 3). Inasmuch as the Ironwood and Biwabik formations are 

 believed to be equivalent, this conglomerate may indicate a widespread discon- 

 formity. However, this disconformity is of very minor significance as compared 

 with the profound unconformities which separate the Upper Huronian from the 

 Animikie and the Animikie from the Lower Huronian, and, so far as known, has 

 little importance in the consideration of the major subdivisions of the Huronian series. 



^ Monograph ig, U.S. Geol. Survey. 



