VOLCANICS OF THE MICHIGAMME DISTRICT. 509 
angular shape stand out well from the brownish red matrix. A 
fresh surface shows the fragments to be light green, and the 
cement a darker green. The fragments vary in size from bowl- 
ders to minute ones which fill in the interstices between the 
larger ones and thus serve to make the mass compact. They are 
for the most part not amygdaloidal, but very dense, although 
those which are amygdaloidal and even scoriaceous do occur 
mixed with the dense fragments, and in places equal them in 
quantity. The basalt and andesite appears to be equally well 
represented in the pyroclastics. 
These deposits show in places a well-developed banding 
caused by interbedding of layers in which coarse and fine frag- 
ments alternately prevail, illustrating well the varying intensity 
of the volcanic discharges. Gradation could also be traced 
from the coarse breccias to the delicately banded portions which 
are exactly analogous to the fine sand and ash beds of the mod- 
ern Tertiary and recent volcanoes. These fine sand and ash 
beds are not, however, composed of crystal elements, but as 
far as the observations go are composed solely of very fine 
rock fragments. In such places the difference between the 
thickness of the ash beds and of the tuff is naturally very great. 
The average thickness of the ash beds observed was about three 
rect. Ii Whe SANS EXYPOOSUIME UNS CUES AS itOMN FO tO TOO treet 
thick. The thickness of the more massive layers could not be 
determined. Especially good opportunities for observing the 
varying relations between tuff and ash beds is offered by the 
third cut on the Milwaukee & Northern Railway west of Bal- 
sam, Michigan; south of the lake in Sec. 5, Township 43 N., 
Range 32 W., Michigan, the tuff and lava flows are very well 
exposed. The pyroclastics seem to predominate in the north- 
western portion of the region in the neighborhood of Amasa. 
It is almost needless to state that these pyroclastics have 
undergone a great amount of alteration, and yet the thin sec- 
tions of some of the rock fragments are the freshest and prettiest 
seen. The changes which have taken place were purely of a 
metasomatic nature, and as water is the chief agent in such 
changes, these began in the interstices. In the case of the frag- 
