Pirsson — Petrography of Tripyramid Mountain. 405 



Art. XXXV. — Contributions to the Geology of New Hamp- 

 shire, No. V ; Petrography of Tripyramid Mountain • by 

 L. V. Pirsson. 



Introductory. — In a previous paper in this Journal* Prof. 

 Win. North Rice and the author have described the geology 

 of Tripyramid Mountain and discussed its structure and prob- 

 able origin. It was there shown that the mountain was prob- 

 ably laccolithic in nature, and concentric in the arrangement of 

 the different rock -types composing it. The outer border against 

 the granite of the region is formed in considerable part of a 

 gabbro, which is succeeded by a broad inner shell of monzonite, 

 and this in turn encloses a core of syenite. In one place the 

 gabbro is separated from the monzonite by a narrow zone of 

 norite. In the gabbro and outer granite a few narrow dikes of 

 lamprophyric character were observed, while with small aplite 

 dikes and stringers all the rocks of the complex are reticulated. 

 For further details the reader is referred to that article ; the 

 present paper is devoted to the description of the rock-types 

 mentioned above, and to the consideration of their petrographic 

 relations. Outside of various references to their mineral com- 

 position and megascopic appearance by Hitchcock, f the only 

 investigations which have been made of these rocks appear to 

 consist in a chemical study of the constituents of the gabbro by 

 E. S. DanaJ and an examination of it in thin section by G. W . 

 Hawes,§ and these studies are referred to later. 



In this paper the rocks will be considered in the following 

 order : syenite, monzonite, norite, gabbro, aplite and lampro- 

 phyres. 



Syenite, var. Umptekite (Nordmarkose). 



A comparison of specimens and of sections shows great uni- 

 formity in this rock in all the exposures seen. The freshest 

 material was found in a block which had fallen from the out- 

 crops above down on the South Slide, and this was therefore 

 selected as the type for description and analysis. 



Megascopic. — Phanerocrystalline ; medium grained ; pale 

 flesh-colored ; dominantly feldspathic, but dotted with black 

 anhedra and prismoids of hornblende 2-4 mm by l mm in size ; 

 feldspars mostly equidimensional, l-5 mm ; the great majority 

 composed of flesh-colored orthoclase, but mingled with them a 

 considerable quantity of grains of a white sodic feldspar ; gran- 



*Vol. xxxi, p. 269. 



f Geology of New Hampshire, vols, i and ii. 



% Composition of the Labradorite Eocks of Waterville, this Jour. (3), 

 vol. hi, p. 48. 



§ Geol. of New Hampshire, vol. iii, pt. 4, Mineralogy and Lithology, by 

 G. W. Hawes, p. 166, 1878. 



Am. Jour. Sci.— Fourth Series, Vol. XXXI, No. 185.— May, 1911. 

 28 



