HOOSAO MOUNTAIN. 67 



ing gneisses, but also a marked columnar jointing. The form of the hills 

 and the very existence of this topographical belt seem due to the rapid 

 erosion of these rocks. Their held relations show that they are of intrusive 

 origin — dikes, in fact, injected parallel to the strata and then crumpled and 

 metamorphosed — and their microscopical characters agree with those of 

 similar rocks, described by Lossen, Teale, and many others, which have been 

 recognized as altered dikes. They correspond in part to the "metamorphic 

 diorites" of Hawes. 1 The}' are briefly described by President Hitchcock 

 in the Geology of Vermont, Vol. ir, p. f>78, where the remark is make that 

 they "may be only huge dikes." 



Fig. 26.— Crumpled amphibolite, Mount Holly, Vermont. Natural size 



The white bands are feldspar, the dark hands hornblende principally. The vertical groovinga which coincide with 

 the line of apices of the folds (the specimen standing as m nature) show but faintly in the figure, and are doubtless caused 

 by rain flowing over the vertical surface and following the depressions between the small folds. 



Iii the hand specimen we see a dark heavy rock, with very faint parallel 

 structure in the coarse varieties. Studied in thin section these rocks 

 have very uniform characters; the least altered forms, of coarser grain, 

 are composed of crystalloids of hornblende and rounded grains of plagio- 

 clase feldspar. The hornblende is a massive brownish-green variety in 

 short irregular crystalloids, the central parts of which are filled with a dark 

 opaque substance, which, with high powers, is resolved into a mass of little 

 crystals of rutile; they sometimes inclose crystals of apatite. In some 



1 Litliology of New Hampshire, ]>. 225 



