IOWA ACADEMY OF SCIENCES. 
71 
Faults . — At the reservoir in Nashua are evidences of a fault; 
there is in the slate a seam 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 MetamorpMsm. — 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 explanation. 
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. 
