136 COPPER-BEARING ROCKS OF LAKE SUPERIOR. 
and distinctly pre-existing cavities. The amygdules vary very greatly in 
abundance. Sometimes, as in the upper amygdaloids of the Montreal River, 
and, yet more strikingly, in some of the minutely vesicular amygdaloids ot 
the Minnesota shore in the vicinity of Agate Bay, they nearly exclude the 
matrix, showing that the original rock must have been as vesicular as a 
sponge. The amygdules at times take on a cylindrical form, with the axis 
at right angles to the bedding, sometimes extending in this way to a length 
of several inches. I have noticed such ‘spike amygdules” in a number of 
cases in the thin basal amygdaloids. Again, the amygdaloidal cavities are 
found elongated laterally in a common direction, this being carried some- 
times to such an extent that the amygdules are thinned to mere strings. 
This is to be finely observed in some amygdaloids on the lake shore at 
Duluth, as well as at a number of points on the coast further to the east- 
ward, and, in general, some traces of this elongation are more often to be 
made out than not. It suggests a flow of the vesicular matrix while in a 
viscous condition. 
The internal alterations that these amygdaloids have commonly under- 
gone have been described in some detail in the last chapter. The altera- 
tion has in some cases been an extreme one; ‘large parts of the bed have 
lost their amygdaloidal character, and now consist of quartz, epidote, calcite, 
prehnite, chlorite and decomposed amygdaloid associated in the most irreg- 
ular manner. Such are the beds worked for copper on Keweenaw Point,” 
and similar beds occur throughout the entire extent of the series. 
A kind of amygdaloid very interesting in its structural relations is to be 
seen largely exposed along the Minnesota shore between French and Split 
Rock rivers, in the Agate Bay Group of beds.? This is a highly vesicular, 
true amygdaloid, occurring in two phases which graduate into each other. 
In the one a crumbling light-brownish matrix holds large amygdules of 
radiating laumontite, to which zeolite the matrix itself has often partly 
altered. In the other phase a reddish-brown, iron-stained, hard matrix in- 
cludes thickly studded minute amygdules of saponite and laumontite. The 
point of especial interest with regard to these amygdaloids is their stratiform 
1R. Pumpelly, Geology of Wisconsin, Vol. III, p. 32. 
2See chapter VII, pp. , 
