STRATIFORM AMYGDALOIDS. 137 
condition. They lie in layers one inch and upwards in thickness. The 
layers are quite irregular and non-continuous, but affect the whole mass: 
This structure is brought out with especial prominence on weathered cliff 
sides, the thinner layers falling away in fragments as from a cliff of shaly 
limestone. Occasionally seams of red sandstone are interleaved or overlie 
a thickness of the stratiform amygdaloid, into which the sandstone pene- 
trates at times through fissures and irregular openings. This amygdaloid 
graduates laterally into kinds without stratification. It occurs in thick- 
nesses running from less than a foot up to as much as 15 or 20 feet, but 
the usual thickness lies between 5 and 8 feet. Interbedded with and grad- 
ing into these amygdaloids are massive layers of fine-grained olivine-dia- 
base or melaphyr with a quite pronounced vertically columnar structure. 
A fine show of both is to be seen on the west side of Agate Bay, on the 
Minnesota shore, where a total thickness, measured, of 101.5 feet included 
seven massive layers running from 2 to 15 feet in thickness, and seven 
layers of stratiform amygdaloid running from 3 to 20 feet in thickness. A 
single seam of red shaly sandstone 2 to 3 feet in thickness is included. 
The resemblance of these amygdaloids to beds of sedimentary origin, 
however striking at the first glance, is nevertheless lost on close inspection, 
for the vesicular structure is seen to be identical with that of the ordinary 
amygdaloids, while no trace of a fragmental nature can be detected either 
with the naked eye or in the thin section with the microscope. The thin 
section shows a completely interlocked crystalline texture, and a composi- 
tion precisely the same as the underlying massive rock, except as to the al- 
terations and amygdules, and the presence of a considerable amount of 
unindividualized altered magma substance. The stratiform condition may 
be in part due to a succession of thin scoriaceous flows, and in part to a 
true “‘fluidal” structure. It appears often to result from an arrangement 
of amygdules more plentifully on certain planes, subsequent molecular 
changes bringing out an apparent stratification by following the planes on 
which the amygdules were thickest. But, however this may be, there can 
be no question as to the identity of origin of these amygdaloids with all 
others of the series, although in Owen’s Geological Survey of Wisconsin, 
