TOE NORTHFIELD SEMIS YNCLINB. 213 



ends from friction ag-;iinst tlie wall of the fault on the south; for an inspec- 

 tion of the map will show that to the north of this transverse fault the 

 strike of all the beds bends from a nortli-south direction i-ound to an 

 easterly direction. It is remarkable, also, that to the north of this fault all 

 the beds of the western flank of the syncline, as well as the remnant rif the 

 eastern flank, are inverted and now dip uniformly to the west. 



The topography of the region is to an exceptional degree dependent 

 upon its geological structure. Each of these transverse faults is now the 

 gorge of a bi'ook. 



The upper beds of the seines — tlie Conway mica-schists — are the most 

 resistant to erosion, and form the high hills, which are pushed forward or 

 recede as the block of mica-schist of which each is made is jjushed forward 

 or back by the faulting. The amphibolite is more rapidly eroded, and it 

 forms a deep furrow across the town, in which runs what is appropriately 

 called the Gulf road, the word gulf being used in this sense in several 

 places in western Massachusetts. The basal quartzite is also resistant and 

 mantles over the gneiss of Bnisli and Crag mountains in sharp, angulai- 

 ridges, which can be seen and recognized so far off as the station at Millers 

 Falls as peculiar and not like the forms of the gneiss. The Gulf road men- 

 tioned above runs south from Northfield to Erving, at the east base of Brush 

 and Crag mountains, and continues a long way on the hornblende-schist of 

 this series, and here the whole may be best studied. 



THE GULF ROAD SECTIONS. 



Two miles south on tliis road a side road goes up onto the mountain 

 westerly to the house of Mrs. J. Robbins, and a little farther south a similar 

 blind road runs east to the house of R. H. Minot. The whole series is 

 well exposed along tliis line, and it is described in the following section, 

 beginning at the west end: 



The granitoid biotite-gneiss («), which makes the mass of Brush 

 Mountain, forms the base of the section. It is the northern portion of the 

 large Pelham area of the Monson (Cambrian) gneiss. The line of boundary 

 between the basal quartzite (the Rowe schist) and this gneiss runs beneath 

 the Robbins house, making a large curve to the east, and tlie two rocks are 

 unconformable. This is shown by the fact that the gneiss has strike N. 

 40°-50° W., dip25'=-3o^ E., while the (juartzite above has strike N. 15"' W., 



