380 /. MORGAN CLEMENTS 



This hornblende, with respect to its rather exceptional pleo- 

 chroism and its general characters, seems to agree very well 

 with that described by van Horn 1 from very similar rocks from 

 Italy, and like that is possibly very basic. The brown horn- 

 blende possesses a further interest in that it is frequently ren- 

 dered very dark by the number of exceedingly small inclusions 

 within it, and in this, and also in its color, resembles so strongly 

 hypersthene as to be readily mistaken for it upon cursory exami- 

 nation. The inclusions referred to are determined to consist 

 of characteristic heart-shaped and geniculate twins of rutile, 

 and pointed pyramidal crystals of octahedrite. Others show a 

 flat tabular development, somewhat similar to that of brookite, 

 though these could not be positively determined as that mineral. 

 Associated with the above were numbers of hexagonal, clove- 

 brown plates appearing in cross section as sharp lines. A 

 gradation between these plates and large masses of ilmenite was 

 traced. It thus appears that these inclusions are all titaniferous 

 minerals. 



The second kind of hornblende is the compact, strongly 

 pleochroic common green hornblende, and the third kind is a 

 non-compact reedy variety of light green hornblende. This 

 last is probably secondary, but secondary after the original horn- 

 blende, thus not affecting essentially the character of the rock. 



The pyroxene is represented by a monoclinic variety and by 

 the orthorhombic bronzite. The presence of olivine was deter- 

 mined with considerable doubt. None of the remaining minerals 

 show anything of special interest. 



The leading essential constituents described are combined 

 in variable quantities, and accordingly a number of different 

 mineralogical types of rocks are produced. The important 

 types which will be described are hornblende-gabbro, consisting 

 essentially of hornblende and labradorite ; gabbro, consisting of 

 monoclinic pyroxene and labradorite; and bronzite-norite, com- 



1 Petrographische Untersuchungen tiber die Noritischen Gesteine der Umgegend 

 von Ivrea in Oberitalien, by F. R. van Horn. Tsch. Mitt., V Heft, 17 Bd., 1897, 

 p. 409. 



