Pirsson and Rice — Geology of Tripyramid Mountain. 275 



used in the former, or in the later, petrographic sense, as will 

 be shown later. He speaks also of the presence of trap dikes 

 in it from an inch or two up to a foot in thickness. He does 

 not mention their color, so it is uncertain whether this refers to 

 aplitic or lamprophyric dikes, or to both, but the use of the 

 word trap suggests the latter. He speaks also of extensive 

 layers of black hornblendic rock a mile below the slide on the 

 stream ; this evidently refers to the gabbro mentioned later. 



The next account bearing on Tripyramid is found in Hitch- 

 cock's Geology of New Hampshire.* In this, references to 

 the mountain, to its rocks and geology, are made in a number 

 of places, and in vol. II, p. 211 and following, a general de- 

 scription of its geology is given. As we shall have occasion in 



Fig. 4. 



Fig. 4. Generalized section of Tripyramid according to Hitchcock 

 (fig. 19). 



several places to refer more specifically to Hitchcock's Avork 

 we shali content ourselves here with a brief summary of his 

 account. It should be borne in mind that in the seventies, 

 when this was written, certain views in regard to the origin and 

 relations of rocks that we now regard as unquestionably 

 igneous, such as granite, were then prevalent, and had great in- 

 fluence in his interpretation of the geology of the White Moun- 

 tains. We may add also, that where through a knowledge of the 

 local geology we are able to disentangle the facts, observed in 

 the field by Hitchcock himself, from these hypothetical views 

 and from the observations and views of other people, not infre- 

 quently incorrect, with which they are more or less mingled, they 

 are generally confirmed by our own studies. Hitchcock's under- 

 standing of its geology is most easily explained by the aid of 

 the generalized section which he gives and which we have repro- 

 duced in fig. 4. His results were obtained by a traverse up 

 Slide Brook, whose rock bed had been largely laid bare by 

 the then recent avalanche. This stream he calls Norway 

 Brook, for a reason mentioned later. He found the porphyritic 

 granite of the lower valley floor succeeded by an area of a labra- 

 dorite feldspar rock, which is called "ossipite"; this is succeeded 

 by "syenite," •which changes in character as one proceeds 



* The Geology of New Hampshire, by C. H. Hitchcock, 3 vols, 1877. 



