OLIVINE-FREE DIABASE. 65 
sends out long arms, which surround or penetrate the ad joining primary constituents. 
In this manner the chlorite-like pseudomorphs after plagioclase and pyroxene, ete., are 
sometimes incorporated into these pseudo-amygdules. Very often one of these bodies 
has a large central area filled with closely-packed radiating spheres, surrounded by 
fragments of a once continuous band with cross-fibrous structure, which evidently 
once formed the outer limit; outside of these fragments is an outer chlorite area, 
resembling that in the center, and generally bordered on its outer limits by a narrow 
cross-fibrous band which adapts 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 inclosing 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 secondary 
mineral, which was attacked at the same time around its circumference, producing the 
outer band (shell), and throughout the interior. 
Amygdules resulting from both these processes are abundant in the amygdaloids 
proper; but they betray their origin in a marked manner, and differ essentially from 
these pseudo-amyegdules. 
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 transitional form destined to be changed to spheres 
with radiating structure. 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-fibers, instead of being parallel to each other, are more 
nearly perpendicular to the axial line of the band, and form closely-packed groups, in 
each of which the fibers radiate from a central point on the axial line, forming minute 
hemispheres, which bristle towards the interior of the body. The next stage of growth 
finished the other half of each sphere, and what was a cross-fibered band becomes now 
indistinguishable from the rest of the central filling, a new band having formed out- 
side of the previous one. In places we find perfectly straight bands, with actually 
parallel cross-fibers, which can hardly be supposed to break up into spheres; and, 
indeed, we find that new parallel bands are formed outside of these 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. 
The fine-grained olivine-free diabases are typically developed in beds 
22, 69 and 87 of Marvine’s Eagle River section, Keweenaw Point;? at the 
old Union minein the Porcupine Mountains; at the upper falls of the Montreal 
River, where the exposures are very large; and at numbers of other points 
on the South Shore. Similar rocks, apparently, are largely developed on 
Isle Royale, and again on Michipicoten, though this has not been proved 
by microscopic study. On the Minnesota coast, however, though occurring, 
‘Geological Survey of Michigan, Vol. I, part II., Chapter VII. 
5LS 
