AMYGDALOIDS AND PSEUD-AMYGDALOIDS. 135 
olivine, and have the amygdaloids more strongly developed. However, 
the olivine-bearing kinds are often provided with very highly vesicular 
amygdaloids, as for instance most of the succession of beds seen on the 
Minnesota coast between Knife River and Split Rock River, described on 
a subsequent page under the name of the Agate Bay Group. 
The amygdaloidal and compact portions appear to grade into each 
other through an intermediate stage, in which the amygdules are less plen- 
tiful. This intermediate stage, recognized by Pumpelly and Marvine in 
their descriptions of Keweenaw Point geology as ‘‘amygdaloidal mela- 
phyr,” the first-named geologist has since shown to be essentially different 
from the upper amygdaloid, in that in the latter most of the amygdules fill 
sharply defined pre-existing cavities, while in the former they occupy the 
positions of primary rock constituents. A threefold division of these beds 
into true amygdaloid, pseud-amygdaloid, and compact portion is thus to be 
recognized. In a number of cases I have myself observed still a fourth 
division, viz: a true amygdaloid, occupying the base of the bed. This has 
little or no gradation-zone into the overlying compact portion, is thinner 
than the top amygdaloid, and shows sparser and larger amygdules, which, 
moreover, occupy cavities whose walls are unusually dense and sharply 
defined. All of these divisions are often present, but one, or all save one, 
may fail. When one division only is recognizable, it is commonly the massive 
portion, but in the thinner beds the pseud-amygdaloid or alteration-zone 
not unfrequently extends all the way to the base, there being then no 
massive unaltered portion. 
The amygdaloidal portions of these beds present many complicated 
and much varied phases, the complexities arising chiefly from molecular 
alterations subsequent to the solidification of the rock, from admixtures of 
sediment, or from both of these causes at once. Still, by a study of fresher 
conditions, it is easy to recognize certain constantly recurring main charac- 
teristics, viz: a matrix always different from that of the more compact por- 
tion of the bed, in that it is much denser and often much less perfectly 
crystalline, and always much more prone to alteration; and amygdules of 
one or more of calcite, chlorite, quartz, epidote, prehnite, laumontite, cop- 
per, orthoclase, or the alteration-products of these, filling sharply defined 
