506 Pirsson and Washington — Geology of New Hampshire. 



Megascopic. — Color, dark stone-gray ; of very fine grain ; 

 strongly foliated ; even texture ; highly micaceous and splitting 

 readily along the plane of chief fracture ; showing with lens 

 an even mixture of white granules and flakes of mica. 



Microscopic. — The minerals seen in the section are quartz, 

 orthoclase, biotite, plagioclase, sillimanite, garnet, muscovite, 

 apatite, iron ore and zircon in the order of their importance. 

 The quartz is in irregnlar elongated granules, filled in places 

 with dusty specks, shows occasional lines of fluidal cavities and 

 rarely any evidence of undulatory extinction pointing to dis- 

 turbance of the optical system by strain. The orthoclase has a 

 similar form and in places is filled with shreds of sericitic mus- 

 covite ; generally it is clear and unaltered and shows no twin- 

 ning. The biotite is in small brown pleochroic flakes, well 

 scattered but tending to lie with the base in the plane of schis- 

 tosity, thus promoting the easy fracture. It sometimes contains 

 iron ore with pleochroic halos. The plagioclase is similar to 

 the orthoclase but distinguished by the albite twinning ; it 

 appears to be an oligoclase. It may be remarked here that the 

 association of the sillimanite suggested the possible presence of 

 cordierite, but careful search failed to reveal it by any of the 

 usual diagnostic characters which it possesses. The sillimanite 

 is in the usual characteristic bundles of needles, and it is not 

 everywhere scattered through the rock but appears distinctly 

 in layers ; although these wedge out in places they soon recur 

 again, running along at about the same horizon. The garnet, 

 of an ordinary character, is in round grains of relatively con- 

 siderable size associated v^ith these sillimanite bands, which 

 often curve around it. The apatite and zircon lie scattered in 

 the quartz-feldspar layers ; they are very small and a signifi- 

 cant circumstance is the fact that they show no crystal outlines 

 but are rounded ovoid bodies. 



In the specimen taken near the contact at Locke's Hill the 

 appearance of the biotite and the fabric of the rock suggested 

 a distinct approach to the characteristic contact hornstone tex- 

 ture, but elsewhere this is not noticeable. 



Origin of the Gunstock Gneiss. 



The characters which have been mentioned above clearly 

 point to a sedimentary origin for this gneiss. There is no 

 sign of shearing or granulation and the minerals show no opti- 

 cal strain — it has therefore been recrystallized. The presence 

 of the sillimanite-garnet bands points to fine clay .layers of a 

 somewhat marly nature interspersed among those forming the 

 main mass of this very fine-grained arkose. The rounded 

 forms of the apatite and zircon grains would point to their 

 having been rolled in the fine sand ; in the metamorphism 



