272 PROCEEDINGS OF THE AMERICAN ACADEMY 



outside of these fragments is an outer chlorite area, resembling that in 

 the centre, and generally bordered on its outer limit by a narrow cross- 

 fibrous band which adajits itself closely to the primary constituents. 

 The greater number of these bodies seem to have resulted from a gradual 

 change of the primary minerals into chlorite by progress from molecule 

 to molecule. At the first glance, the structure does not seem to confirm 

 this view; for the narrow outer band enclosing a large central filling 

 seems to suggest either, 1st, a pre-existing cavity, on the walls of which 

 the thin outer layer was deposited as the older member and, within 

 this, the central filling as the younger ; or, 2d, the replacement by 

 chlorite of a former secondaiy mineral, which was attacked at the 

 same time, around its circumference, producing the outer band (shell), 

 and throughout the interior. 



Amygdules resulting from both of these processes are abundant in 

 the araygdaloids proper ; but they betray their origin in a marked 

 manner, and differ essentially from these pseudo-amygdules. 



Whatever the chemical nature of the process resulting in these 

 pseudomorphs, the central area is the oldest member, while the outer 

 band is the younger ; and its cross-fibrous structure is only a tran- 

 sitional form destined to be changed to spheres with radiating f-tructure, 

 If we examine the structure of the outer band, we find that its line 

 of contact with the primary minerals, or its axial line, is usually more 

 or less serpentine, and that the cross-fibres, instead of being jjarallel to 

 each other, are more nearly perpendicular to the axial line of the band, 

 and form closely packed groups, in each of which the fibres radiate 

 from a central point on the axial line, forming minute hemispheres, 

 which bristle towards tlie interior of the body. The next stage of 

 growth finished the other half of each sphere, and what was a cross- 

 fibred band becomes now undistinguishable from the rest of the central 

 filling, a new band having formed outside of the previous one. In 

 places we find perfectly straight bands, with actually parallel cross 

 fibres, which can hardly be supposed to break up into spheres ; and 

 indeed we find that new parallel bands are formed outside of tiiese 

 until the line of attack becomes crooked, when the normal mode of 

 growth is re-established. The remnants of these straight bands are 

 then preserved in the interior of the body. 



Bed No. 96. — There are two other instances of pseudomorphs 

 after plagioclase, which occur in the lower zone; viz., prehnite and 

 quartz. 



Bed No. 96, lying north of the ''greenstone," in the Eagle River 

 Section, is a rather coarse-grained diabase. It is dirty green, filled 



