GABBEO AHD JfOEITE mTRUSIVES. 235 



two kinds of hornblende are presumed to be primary. The thii-d variety is 

 secondary, but secondary after the original hornblende, thus not affecting 

 the character of the rock. 



The first variety, the reddish-brown hornblende, occurs in the gabbros 

 in anhedra. A zonal structure is marked by brown hornblende occupying 

 the centers of the crystals and by dull green hornblende, which agrees 

 optically with the brown, occupying the outsides. The brown hornblende 

 is somewhat lighter colored than basaltic hornblende. The pleochroism is 

 strong in the following colors: Brown hornblende: a, light yellow or red, 

 with tinge of green; h, red brown; c, same or darker red brown, excep- 

 tionally yellowish-brown; C^I)>a. Green hornblende: a, greenish- 

 yellow; h, yellowish- or brownish-green; C, dull olive green, frequently 

 with bluish tinge; C>T)>a. 



This hornblende, with respect to its rather exceptional pleochroism 

 and its general characters, seems to agree very well with that described by 

 van Horn from very similar rocks from Italy, and, like that, is probably a 

 very basic hornblende.^ Twinning parallel to 100, oo P oo, is very common. 

 An imperfect parting parallel to the orthopinacoid 100, ao P oo, was also 

 observed in some cases. It is also indicated by the platy inclusions which 

 lie in this plane In some of the sections where the green hornblende is 

 not intergrown with the brown the green kind shows very commonly a 

 system of fine striations parallel to the positive orthodome 101, P oo. In rare 

 cases the brown hornblende is intergrown with almost colorless hornblende, 

 one end of a crystal being brown, the other faintly yellowish. Irregular 

 mottled intergrowths of the two were also found. 



The normal brown-green hornblende is rendered poikilitic in some 

 specimens by a few rounded grains of perfectly fresh pyroxene, and also by 

 plagioclase crystals which it includes. This same kind of hornblende is 

 frequently rendered very dark by the number of exceedingly small inclu- 

 sions which it contains, and in this, and also in its reddish-brown color, 

 resembles so strongly many hypersthenes as to be readily mistaken for them 

 upon cursory examination. These inclusions are of several kinds, all dis- 

 tributed throughout the same individuals. It is impossible in studying them 



' Petrographische Untersuchungen iiber die noritischen Gesteine der Umgegend von Ivrea in 

 Oberitalien, by F. K. van Horn: Tschermaks mineral Mittheil., Vol. XVII, 1897, Part V, p. v09. 



