268 Pirsson and Washington — Geology of Red Hill, N. H. 



green and yellow-brown colors and they may vary consider- 

 ably in the same crystal ; the interior being of a lighter color 

 with deeper-toned rims. The pleochroism is very strong ; a, 

 light yellow-brown, b, deep olive, at times practical!} 7 opaque ; 

 c, deep olive-brown to olive-green ; the absorption is very 

 marked, b>C>a. The angle of c A c is large, 30° or more, 

 refractive index high, double refraction very low. Twinning 

 on 100 observed. In some sections the green color predomi- 

 nates but always with a somewhat olive tone. It very much 

 resembles hornblendes in the alkalic rocks of the Highwood 

 Mts. described by the author.* In Brogger'sf table of the 

 alkalic hornblendes there are three groups, the barkevikites 

 with brown colors with small angle of c,\ c and greatest absorp- 

 tion in c, the cataphorites with reddish colors, large angle c^c 

 and absorption greatest in b, and the arfvedsonites, greenish- 

 blue colors, angle c^c very large (75°+) and greatest absorp- 

 tion in a. The present type does not agree with the 

 statement of any of these but shows apparently properties 

 intermediate between all three. So also do the Highwood 

 hornblendes. 



In this connection the writer cannot help noting that it 

 would be a great gain in the accuracy of petrographic work if 

 there were only some definite and accepted standards of color 

 (transparent colors) for comparison. Thus Brogger describes 

 cataphorite in the grorudite from the quarry at Grussletten by 

 Grorud as follows : b, brown-red or deep wine-red, nearly the 

 color of a thick layer of smoky topaz (rauchtopaz) ; c, light 

 grayish to reddish-yellow; a, light greenish-yellow to greenish- 

 blue. In specimens of this rock from the above locality 

 which I had the pleasure of collecting under Prof. Brogger's 

 guidance, the sections show- the cataphorite exactly as he has 

 described it, and I should give the colors as follows : b, brown 

 with a tone of yellow r ; c, pale olive-browm or pale olive-green ; 

 a, pale yellowish-brown ; absorption b>c>o. The character- 

 stic thing about this hornblende to me is its pure browm 

 colors in which I see no tone of red. And wine-red to me would 

 be the color shown by an almancline garnet, while the color of 

 smoky topaz would be a brown or yellow-brown, which Dana 

 calls wine-yellow, a term which describes also the cataphorite. 

 This illustration, which is in no way intended for a criticism, is 

 merel} 7 introduced to show how differently petrographers de- 

 scribe the same colors and the difficulties such differences may 

 occasion. 



In fact the comparison of the two minerals shows to the 

 writer that the hornblende of the Red Hill rock is probably 

 near cataphorite, at times inclining to arfvedsonite. 



* Bui]. U. S. Geol. Surv. No. 237, pp. 66 and 95, 1905. 

 f Grorudit-Tinguait Serie, 1894, p. 37. 



