24 green mountains in Massachusetts. 



below can be explained only by introducing an overthrust fault or by sup- 

 posing that the inverted anticline was pushed <>ut thus far without rupture. 

 The former explanation seems the more likely one and accords better with 

 the thickness of the ( lambrian gneiss and the dips in the schist observed in 

 the tunnel. The bed of white gneiss — 600 to 800 feet thick — when ex- 

 posed to the great thrust which overfolded the lloosac rocks, would, it 

 seems, be less able to adapt itself by minor foldings than the more readily 

 yielding schist, and would be more likely to find its compensation in a 

 rupture and an overthrust fault. 



At the tunnel line the axes of the folds are still pitching to the north. 

 Immediately north of the limestone is a mass of folded lloosac schist, under 

 which the limestone is carried by the pitch of its folds and which is seen at 

 several points to l>e younger than the limestone. The zone of lateral tran- 

 sition is also carried under this hill, and this fact explains the peculiar areal 

 geology of this part of the map (PI. i) on which the color for the Stock- 

 bridge limestone extends along the west side of the schist hill, that for the 

 Vermont formation along the east side. The obscurity disappears on PI. n, 

 where I have separated these transitional rocks from the quartzite and given 

 to them and to the lower part of the limestone a separate representation as 

 Cambrian. 



It is not easy to determine the extent to which overthrust faulting has 

 entered into the building of the Green mountain range in northwestern Mas- 

 sachusetts. Along the eastern side of the pre-Cambrian core of lloosac 

 mountain the movement flattened the coarse pebbles of the conglomerate 

 and granulated their quartz and large feldspars to the point of obliteration. 

 But the great Cambrian conglomerate-gneiss bed as it curves around the 

 core shows no break due to faulting. It is not until we come to the west 

 side of the pre-Cambrian gneiss-core that we find evidence of a rupture in 

 the flat fold, where the hard Cambrian white gneiss has been pushed along 

 an overthrust fault onto the younger schists as tar as the west shaft. Here 

 it seems probable that the rupture was favored by the tact that the troughs 

 of the folds, both above and below the middle limb, were on the lee side 

 of the less yielding pre-Cambrian core, as will be seen from the section 

 (PI in, d). Now this is the same fold that incloses the trough of schist all 



