t^ETROGRAPHY 385 



niatic sections of hornblende with the cleavage planes of both minerals 

 coinciding. It is very pale yellow in color, and has an extinction angle 

 of 40 degrees. It has altered considerably along the cleavage planes, 

 and lines of basal parting, to a yellow or brown serpentinous material 

 with granular structure, low double refraction, and aggregate polarization. 



Biotite is subordinate in amount to the augite ; it is brown and strongly 

 pleochroic in deep brown and pale yellow tones with a greenish tinge. 

 With a few exceptions it seems to have been one of the first materials to 

 crystallize out, and occurs intergrown with the hornblende or as inclu- 

 sions in the feldspar. When included in the latter, small idiomorphic 

 forms are common both in prismatic and basal sections. In some in- 

 stances the biotite shows that strong allotriomorphism toward the feld- 

 spar, which has been previously noted in the hornblende (see plate 34, 

 figure 1). Though generally quite fresh, a few individuals have par- 

 tially altered the chlorite along the cleavage planes, while others have 

 been partially or wholly bleached to muscovite with a faint greenish 

 tinge and higher double refraction. 



Quartz is present in but small amount and fills angular spaces be- 

 tween the feldspar individuals. Fluid and opaque inclusions are very 

 numerous in some grains. 



The following accessory minerals exhibit but slight variations to their 

 usual mode of occurrence : Apatite in very small amount is present in 

 idiomorphic, prismatic, and basal sections, rounded grains and needles 

 being rare. Colorless zircon occurs in square basal sections, stout prisms 

 with pyramidal terminations, and in large and small grains, the latter 

 being sometimes arranged in radiate clusters. Basal sections show the 

 uniaxial and positive character of the mineral as well as the cleavage 

 parallel to go P, which is distinct. When the zircon is included in the 

 hornblende or biotite it is always surrounded by a pronounced pleo- 

 chroic halo. The early crystallization of both the apatite and zircon is 

 shown by their almost perfect form when included in the magnetite. 

 Brown allanite is a rather common accessory in a few slides. It is found 

 in graphic intergrowth with the feldspar, but usually occurs in rounded 

 idiomorphic forms, included in the hornblende, and is always surrounded 

 by a pleochoric halo. The individuals in oo P+ sections are lengthened 

 along the ortho-axis, and all are much traversed by cracks, the only dis- 

 tinct traces of cleavage noted being parallel to o P. Irregular zonal 

 structure is invariable, and in all cases the interior is of a deeper color 

 than the periplier}^ The pleochroism according to the zones is a = deep 

 brown, almost opaque; t ^ brown, yellow; c= light brown, pale yel- 

 low. Nearly all the magnetite which is in considerable amount is in- 

 cluded in the biotite and hornblende. In the former it sometimes occurs 



LV— Bull. Geol. Soc. Am., Vol. 12, 1900 . 



