474 POSITION AND ORIGIN. 



Another-band of hornblende schist is in the mica schist of Norwich. Other bands exist 

 doubtless in the northern part of the State, besides the one mentioned, but we have not 

 traced them out carefully enough to represent them by colors. Wherever we have found 

 beds of it on any Section it will be noticed in the descriptions of that Section. The horn- 

 blende schist of the Brattleboro and Guilford range is 250 feet thick. 



Mineral Contents. 



This rock contains very few simple minerals, as it is mostly a mineral by itself. The 

 most beautiful mineral ever found in it is garnet. They are small but very distinct, and 

 of a blood-red color. The best localities are in Marlboro and near Dartmouth College, a 

 short distance over the New Hampshire line. Other minerals found in it, are rutile, 

 ilmenite, chlorite, orthoclase, tourmaline, sphene, epidote, and pyrites. 



Geological Position. 



Most obviously the hornblende schist of Vermont has the same geological age as the 

 formations in which it occurs. It is therefore of four ages : 1. Age of the middle range 

 of gneiss. 2. Age of the South Vern on gneiss. 3. Age of the talcose schist; and 4. Age of 

 the calciferous mica schist. 



Origin and Metamorphism. 



We might very properly consider this rock as a variety of gneiss, taking hornblende 

 into its composition ; for feldspar is most commonly present. Sometimes, however, it is 

 composed of hornblende, quartz, and mica in folia, and sometimes it is nearly pure horn- 

 blende. It is generally associated with gneiss, sometimes interstratified with it, but more 

 usually overlying it on an anticlinal, as at Shelburne Falls, in Massachusetts, and in 

 Guilford, as our sections show. Hence it is somewhat newer than the gneiss. 



Dr. McCulloch long ago pointed out examples in which hornblende schist had resulted 

 from the metamorphosis of clay slate. "As far as a single fact can prove such a case," 

 he says, " the origin of hornblende schist from clay slate is completely established by the 

 occurrence, in Shetland, of a mass of the latter substance, alternating with gneiss and 

 approximating to granite. Here those portions which come into contact with the latter 

 become first silicious schist, and ultimately hornblende schist ; so that the very same bed 

 which is an interlamination of gneiss and clay slate in one part, is in another the usual 

 alternation of gneiss and hornblende schist." " It would appear that the fusion of clay 

 slate, whether primary or secondary, is, under various circumstances, capable of generat- 

 ing either the common trap rocks or the hornblende schists." 



McCulloch here supposes the metamorphism produced by dry heat, and the frequent 

 occurrence of hornblende crystals in lava, would seem to favor such an origin of horn- 

 blende. But Bischof has made it quite probable that even in lava the hornblende was 

 not formed at the time of the protrusion of the lava, but subsequently, in the wet way ; 

 and the associations of hornblende schist forbid the idea that it was a volcanic product. 

 It must have been formed in the same way as the schists and gneiss with which it is 

 associated. 



