VOLCANICS OF THE MICHIGAMME DISTRICT. SII 
or are masses which were brought together by the action of 
water, will in some cases have to remain undecided. It is sup- 
posed that the major portion is a true xolian pyroclastic 
deposit. 
That certain of the pyroclastics have, however, been depos- 
ited through the mediation of water is shown by specimens from 
Om NE i270) We Seca o4udhownship 45) Ne eivanee 932) We) neat 
Amasa, which is a true basalt conglomerate. In these rocks the 
pebbles are decidedly rounded, but consist of characteristic 
volcanic fragments, as is shown by the amyedaloidal structure. 
The most of the pebbles are thoroughly impregnated with iron in 
the form of magnetite, giving them a black color. Under the 
microscope the feldspar skeletons are occupied now by chlorite 
and muscovite, and the portions between the skeletons 1s a mass 
of magnetite grains, in rare cases with a little hematite. 
The cement is chiefly calcite and quartz in which is developed 
a large amount of chlorite and muscovite, and in which occurs 
also magnetite, though the last is more common in the frag- 
ments. No pieces of undoubted sedimentary rocks were 
observed in the conglomerate. 
The acid volcanics—TYhese play a very subordinate role, 
occurring in such small quantity as to make it impossible with- 
out very great exaggeration to place them on the accompanying 
map. Their relations to the basic volcanics are obscured by 
lack of exposures, but in no case were they observed as dikes in 
the latter. The trend of the ridges formed by the acid rocks 
agrees with the general strike of the banding in the basic tuff 
deposits, and they are presumed to represent acid flows and tuffs 
interbedded with the basic lavas and pyroclastics. 
On fresh fracture they are black, grading with advancing 
alternation into chocolate brown to reddish rocks. The weath- 
ered surface varies from white to reddish. This has in one case 
brought out very well the fluxion banding of the rock. 
Their structure is very pronouncedly porphyritic, the quartz 
and feldspar crystals standing out plainly from the groundmass, 
which is usually dense with somewhat resinous luster. Under 
the microscope they are found to be typical quartz porphyries, 
