5o8 ERNEST HOWE 



has been found in a number of cases. Professor Williams states 

 that "there is considerable evidence to show that the greenstones, 

 both of the Menominee and Marquette regions, solidified at the sur- 

 face, under subaerial or subaqueous conditions." There are, how- 

 ever, a number of instances where the rocks are clearly intrusive, 

 and it is these that the Irving greenstones most closely resemble. 

 The character of the metamorphism appears to have been the same 

 in both regions, and the rocks from which the greenstones were 

 derived were undoubtedly very similar. The most important differ- 

 ence is that tuffs and surface flows have been recognized in the Lake 

 Superior complex, while no rocks of this nature have been seen in the 

 Irving. Flows or fragmental deposits of igneous origin may, of 

 course, have been originally present, but all traces have been 

 destroyed. 



The Irving greenstones and those occurring near Salida in the 

 Arkansas Valley, some 125 miles to the northeast, possess a number 

 of common characteristics; in fact, the description by Mr. Cross^ of 

 the hornblendic members of the Salida rocks might be applied directly 

 to those in the Needle Mountains. In both localities granular horn- 

 blendic rocks occur and also denser rocks in which the microscope 

 reveals an ophitic structure. The only rocks found in the Salida 

 section which differ greatly from those of the Irving are ones which 

 are in some instances sufficiently well preserved to indicate a struc- 

 ture and composition like that of rhyolite, and which in extreme cases 

 of metamorphism suggest finely mashed micaceous quartzites. There 

 are not sufficient data in regard to the field occurrence of these rocks 

 to make it possible to say whether they were originally surface flows 

 or intrusives, but in regard to the series as a whole, Cross is inclined 

 to believe that they represent "a great series of surface lavas erupted 

 in Algonkian time." 



In a recent monograph of the United States Geological Survey, 

 Bayley^ has redescribed the greenstones of the Menominee region 

 under the name of "Quinnesec schists" and has referred them to 



^ On a Series of Peculiar Schists Near Salida, Colorado. Proceedings of the 

 Colorado Scientific Society, Vol. IV (1893), pp. 286-93. 



2 W. S. Bayley, "The Menominee Iron-Bearing District of Michigan," Mono- 

 graph 46 (1904), U. S. Geological Survey. 



