44 PHYSICAL GEOGRAPHY. 



posed of fragments of slate, evidently derived from this formation. The 

 lower portions adjacent to the slate are chiefly composed of it, and even 

 at the summit small dark pieces, apparently of the same material, abound. 

 A similar rock with dark fragments is found on Twin mountain. The 

 composition of the cement shows it to be allied in character to the felsites 

 elsewhere found overlying the trachytic granite. 



A somewhat similar slate occurs between Mt. Willard and Mt. Field. 

 Specimens from the two localities are not distinguishable from each other, 

 and the mass of Mt. Willard is a trachytic granite. These slaty rocks 

 pass into quartzites, if not into felsites, and cover a considerable area, 

 including the country from Mt. Willey to beyond Mt. Tom, over three miles. 

 Well marked crystals of andalusite are found in a similar slate on the 

 north-east spur of Mt. Tom, which seems to ally the series with the 

 andalusite slates of the Coos group along the head waters of Ellis river, 

 at the east side of Mt. Washington. I observed that jointed planes 

 existed in the trachytic granite parallel with the slaty strata above 

 them on Mt. Willard, like those described upon Pequawket. Passing 

 to the first peak of Mt. Field, the line of union of the granite and slate 

 was traversed, having a compass course of N. 25°.W. In the saddle of 

 Mt. Field the slates dipped 50° S. 20° W. But on the mountains south 

 nothing is found to correspond with the feldspathic and brecciated cap of 

 Pequawket. The relations of this slate to the granite and felsites demand 

 further examination. 



Relative Position. 



A few considerations will serve to indicate the probable relative posi- 

 tions of the rocks that have been described. The sections given of the 

 common granite, trachytic granite, and the felsites, seem to determine 

 their relative positions, the last being at the top. The brecciated granites 

 of Franconia appear to be older than any of these, and to underlie them, 

 as already stated; and hence there may not be any correspondence 

 between them and the breccias made up of felsites and labradorite. If 

 these points are assumed, the porphyritic gneiss can be shown to be at 

 the bottom of the series, for it lies outside of the lowest of them. Two 

 principal ranges of this rock enter the limits of our map. The eastern 



