288 PROCEEDINGS OP THE AMERICAN ACADEMY 



Bed No. 69 of the Eagle River Section consists of fifty-six feet 

 of the lower zone, eleven feet of pseudo-amygdaloid, and six feet of 

 amygdaloid. 



The lower zone is a fine-grained, dirty-green rock with uneven 

 fracture. It is easily scratched ; has specific gravity 2.87-2.95, and 

 the powder yields a little magnetite. 



The thin sections resemble those of the lower zone of bed 87. The 

 plagioclase is much altered, — containing in the freshest many tufts of 

 chlorite — and is often represented only by pseudomorphs of chlorite, 

 and in places these are merged into chlorite pseudo-amygdules. 



The augite is in part very fresh, in part changed to its characteristic 

 pseudomorph. 



The amygdaloid is a very compact, hard rock, with subconchoidal 

 fracture. It consists of very irregularly mixed brown and green por- 

 tions, both hard, the brown abounding in amygdules, from one-third 

 inch diameter down, chiefly of prehnite ; often of prehnite as an outer 

 member, and a central filling of quartz in some, in others calcite. The 

 green contains fewer apparent amygdules. 



Thin sections of the brown part show the sharp outlines of compara- 

 tively large porphyritic feldspar crystals, and of countless long slender 

 feldspar microlites separated by an opaque brown substance. These 

 feldspar forms are now occupied by brilliantly polarizing aggregates of 

 prehnite. 



Splinters of this brown matrix fuse in the flame of an alcohol lamp. 



Some of the feldspar forms contain a large amount of a soft, light- 

 green, seemingly amorphous mineral, which is, probably, pseudomorph- 

 ous after prehnite ; the rest of the pseudomorph in these cases seems to 

 be quartz. 



The amygdules have very sharply defined contours, and form bril- 

 liantly polarizing aggregates of prehnite. Quartz occurs in seams 

 which cut through the prehnite of the matrix, and of the amygdules. 



An examination of thin sections of the green parts shows that they 

 are derived from the brown. They consist still to a great extent of 

 prehnite, and many pseudomorphs of this after the feldspar are visible ; 

 but it is everywhere more or less changed to the light-green, soft sub- 

 stance (of which some was seen in the brown variety), and considerable 

 areas of the field are wholly changed to this substance, which is thor- 

 oughly cut up by curving cracks of irregular shape and size, which are 

 evidently due to contraction, and are now filled with quartz. But little 

 of the brown staining seen in the brown variety is present here: the 

 iron oxide causing it has, perhaps, gone towards forming the green- 



