THE AGATE BAY BEDS. 287 
cite. This seems the order of abundance for the whole group, but in sep- 
arate layers either one or the other of the first two may predominate. The 
small, rounded, white or greenish-white spots of saponite are very char- 
acteristic. Prehnite is a much rarer amygdule, and agate still rarer. An- 
other equally important characterisijc is the stratiform appearance taken on 
by these amygdaloids. This appearance, which has received some notice 
on a previous page, is also found, though to a less extent, in the interbedded 
massive layers, or rather in the massive lower portions of the beds of which 
the amygdaloids form the upper portions. In both amygdaloids and compact 
portions this stratiform appearance is especially brought out by weathering. 
This was seen beautifully illustrated at the mouth of a creek on Sec. 15, T. 
52, R. 11 E., about two and a half miles above Agate Bay. The creek 
enters the bay over the low shore cliff, into which it has worn its way back 
for some distance. Just where the water flows over it the rock is hard and 
massive, without trace of stratiform appearance, but on either side it may 
be traced distinctly into the usual obscurely stratiform material. 
At a little distance an exposed cliff presents much the appearance of a 
series of sedimentary beds, as for instance a set of rather heavily bedded 
sandstones alternating with shales; indeed, these rocks, as already indicated, 
were long ago called ‘“‘metamorphic sandstone and shale” by Norwood, and 
are now regarded as such by N. H. Winchell. But a closer study shows 
that here, as everywhere, both heavier and thinner layers are made up of 
rocks identical with the amygdaloids and diabases of Keweenaw Point, 
whose completely interlocked crystalline condition in the lower portions, 
and highly vesicular condition in the upper portions—the massive and vesic- 
ular parts grading into one another—abundantly prove their origin as lava 
flows. The lower portions of the beds have, too, very frequently, a well- 
marked columnar structure, another characteristic of the Keweenaw Point 
flows, and of flows of eruptive rocks generally. As already said, microscop- 
ically, these rocks are olivine-bearing augite-plagioclase kinds identical 
with the Greenstone of Keweenaw Point, even to the peculiar crowding 
between the augites of the olivine and magnetite particles. They are 
also the same rocks, only finer in grain, as the coarse black diabases which 
Norwood and Winchell themselves regard as eruptive, and—which will be 
