IOWA ACADEMY OP SCIENCES. 71 



Faults. — At the reservoir in Nashua are evidences of a fault; 

 there is in the slate a seaca of graphitic slate with veins of 

 quartz near by. In this graphitic slate much crushing and 

 slipping has occurred. The strata are on edge with strike 

 N. 73" E. 



The argument for a fault in this locality is sustained, in fact 

 made necessary, by the structure of the region. The general 

 succession of strata from southeast to northwest, is gneiss, 

 schist, slate, schist, gneiss, with no evidence of unconformity; 

 but at Shattuck's ledge the gneiss appears in close proximity 

 to the slate, with little chance for schist between. The dip at 

 Shattuck's ledge compared with the dip observed in the schist 

 to the south indicates that the gneiss exposed at Nashua is in 

 an anticline. 



North of the gneiss at the quarry just west of Nashua a fault 

 is possible, but not necessary to explain the structure, if schist 

 not exposed underlies the river valley. While schist occurs at 

 Mine Falls, schistose gneiss occurs two miles farther west with 

 no schist that is exposed to the north, and beyond Runnells* 

 bridge the eastern boundary of the slate area bends southeast- 

 ward across the line of strike. Thus while the evidence of 

 faulting is very marked near Nashua it becomes less marked 

 south westward. 



Other evidences of faulting exist near the mouth of Gulf 

 brook, and just west of Hollis Center. Along this line the pres- 

 ence of slickensides in graphitic slate, with quartz seams near 

 by, indicate that a line connecting these two points is a line of 

 faulting. 



Cause of Metamorphism. — Finally, it remains to ascertain the 

 cause of the metamorphism. This involves a petrographical 

 problem, especially on the gneiss. There is no igneous rock to 

 be found in the area, unless the gneiss itself be of igneous, 

 origin. 



If the gneiss itself is not of igneous origin there may be 

 igneous rock not far below, or not far beyond the margins of 

 the area, though no locality of such minerals as are common 

 where igneous material comes in contact with sedimentary 

 material is here to be found, nor is there any evidence of 

 intense heat. 



Regional metamorphism affords a satisfactory explaration. 

 The intense crumpling of the strata, the steep dip, the bands 

 of quartz alternating with the slate along the margins of the 

 gneiss, with lack of evidence of intense heat in the immediate 

 vicinity, all indicate that the metamorphism is regional. 



