THE IGNEOUS ItOCKS. 515 



are thought to be surface flows, while of course the tuffs are always surface 



rocks. In other cases the sheets appear to be intrusive, where they are 



with difficulty distinguishable from the dikes. Indeed, while it is believed 



that by far the greater number of the narrow bands of schistose greenstone 



associated with the pi-e-Clarksburg sediments are true dikes, it is thought 



that a large number of them may possibly be sheets. The distinction 



between the two, however, is not so important as it would be were the 



sheets effusive ones. 



tuvj sheets. 



The undoubted sheet rocks are exposed in a few places on the surfaces 

 or sides of ledges as a series of bands of a fine, crystalline, green rock, 

 sometimes amygdaloidal on both edges, sometimes on one edge only. 

 These rocks are interbanded with genuine sediments, with ■ which they 

 contbrm in strike and dip. The best exhibition of the bands, which are 

 believed to represent old volcanic flows, is on the little hillock in the NE. ^ 

 sec. 28, T. 47 N., R. 27 W. (Atlas Sheet XXVI), where three greenstone 

 beds are interstratified with graywacke-like quartzites. 



Again, in the cut on the Chicago and Northwestern Railway in the 

 NW. i sec. 8, T. 47 N., R. 26 W. (Atlas Sheet XXXI), about IJ miles 

 southeast of Negaunee station, there is an excellent section exposed 

 tlu-ough seven bauds of chloi'ite-schists that are interleaved with "flag ores." 

 Rominger gives a sketch of the cut in his report,' and describes it as follows: 



The stnita in the cut form two successive anticlinal arches, which are in two 

 places transversely intersected by wedge like masses of chloiitic schists Intruded from 

 below. The upper portion of the ledges is formed of the banded alternating beds of 

 pale brownish jasper and of siliceous ore seams like the Jasper-banded rocks of the 

 McOmber mines. * • * 



As shown by fig. 18, p. 332, the present authors observed but a single 

 anticline in the schist bands, and discovered portions of seven sheets of the 

 chlorite-schists instead of the two pictured by Rominger. The intersecting 

 "wedge-like masses of chloritic schists" were not seen. Only two of the 

 sheets show the complete fold, but the others are so regularly disposed 

 about the axis of this fold that there can be little doubt that the bands on 



' C. Rominger, Geol. Surv. of Michigan, Vol. IV, p. 79. 



