470 POSITION. 



mountain, which all would regard as gneiss, are generally interstratified with other layers 

 of mica or talcose schist and quartz, and the judgment of good observers would differ 

 about the line where the gneiss predominates over the schists ; that is, about the limits of 

 the gneiss. 



It is quite instructive to take the Geological Map of Massachusetts, as given in Hitchcock's 

 Final Report of 1840, and trace the geology of the Green Mountain range, first across 

 Massachusetts, and then, by our map, through Vermont. On the south line of Massa- 

 chusetts, almost the whole breadth of the range, not less than twenty miles, is tolerably 

 well characterized gneiss. Yet as we pass northerly on the line of strike along the eastern 

 margin, the gneiss is rapidly succeeded by mica and talcose schists, and the gneiss be- 

 comes so pinched up as to form a mere wedge before we get across the State, and it is 

 doubtful whether the mica schist does not absolutely cut off the gneiss ere we reach the 

 north line. We incline to the opinion, however, that a narrow belt of Green Mountain 

 gneiss does extend across the whole State into Vermont. At any rate, as our map shows, 

 as soon as we enter Vermont the gneiss again spreads out over a very wide belt, though 

 in many places we are obliged to judge of the subjacent rock by the detritus above it. 

 On its eastern side the gneiss is well charged with feldspar ; but as we go northerly it 

 becomes less so, and the formation diminishes in width, while the talcose schist spreads 

 out wider and wider to the Canada line. Precisely how far north to extend the gneiss 

 has been a point of difficulty, and how wide to make it. Probably what might not inap- 

 propriately be called gneiss may be traced even to the Canada line ; but we meet with 

 little north of Lowell. 



We have expressed the opinion in another place, that gneiss, mica and talcose schist, 

 and even some beds of quartz may be only metamorphic varieties of the same original 

 formation. Their interstratification within the space of a few rods, certainly makes such 

 a supposition probable. And the remarkable changes of these rocks on the line of strike 

 which we have pointed out, strengthens this hypothesis. Indeed, AVC have guessed that 

 all these rocks have been produced by the metamorphism of conglomerate at Wallingford 

 and Plymouth, and surely it is still easier to conceive a like metamorphism from other 

 formations. The conclusion seems forced upon us, that the particular kind of rock result- 

 ing from metamorphism, depended upon the presence or absence of the requisite ingre- 

 dients in the water that penetrated the rocks. It might have the ingredients necessary 

 to produce mica or talcose schist, yet not those which would form gneiss. We can, indeed, 

 see how clay slate and hornblende schist might be changed into gneiss, and probably 

 some of the Vermont gneiss had such an origin. But we may derive it from almost any 

 rock if we suppose it permeated by water containing in solution any ingredients, requisite 

 to form feldspar, that are wanting in that rock. 



We have already made it probable that the calciferous mica schist has been converted 

 into gneiss from Ascutney southward. If so, whatever the age of the schist may be, that 

 of the gneiss is the -same. We have also made it probable that the Green Mountain 

 gneiss forms the center of a folded axis, over which the conglomerate, talcose schist, 

 quartz rock and limestone once mantled. If so, and if we are justified in referring the 

 conglomerate to the base of the upper Silurian, the Green Mountain gneiss, certainly 

 where it forms a folded axis, is probably the oldest of the metamorphic rocks in Vermont 



