VOLEANIGS OF THE MICHIGAMME DISTRICT. 805 
true volcanics could not be doubted for a moment by any one 
seeing them zm szfu or evenin hand specimens. They exhibit the 
vesicular character and flow structure of modern lavas, and inter- 
bedded with them we find great masses of breccia and tuff, com- 
posed of dense and scoriaceous fragments of rock similar in 
appearance to the rocks of the lava flows. A fact to be noted is 
that remarkably few undoubted dikes have been observed. This 
may be due to some extent to the disposition of the strata. 
Both basic and acid igneous rocks are represented in the 
Michigamme district by lava flows and pyroclastic deposits. The 
basic volcanics comprise those altered pre-Tertiary equivalents 
of the basalts and andesites known as melaphyres and porphy- 
rites. In the following pages they are called apobasalts and 
apoandesites. The acid rocks include originally holocrystalline 
porphyritic rocks, quartz porphyries, and devitrified equivalents 
of the rhyolites, which are here called aporhyolites.* 
The apobasalts are by far the most common of the volcanic 
rocks. Their colors are various shades of green, dark olive- 
green prevailing. Upon weathering the green rock is usually 
covered by a thin crust, in which gray, brown, and even pinkish 
tints prevail. 
The lavas vary in coarseness from dense aphanitic rocks to 
those which are medium-grained. No coarse-grained rocks are 
found among the interbedded flows and tuffs.2, Some of the most 
characteristic volcanics are fragments in the pyroclastics. The 
most constant volcanic characteristic is the presence of a well- 
marked amygdaloidal structure, the rounded or more less irregu- 
lar shaped amygdules ranging from those of almost microscopi- 
cal size up to others larger than a pigeon’s egg. Some of the 
rocks are so full of these vesicles as to be truly scoriaceous. 
*F. Bascom: The Structures, Origin, and Nomenclature of the Acid Volcanic 
Rocks of South Mountain. JouRN. GEOL., Vol. I., No. 8, p. 828, Nov.—Dec. 1893. 
?In the southern portion of the area occur great continuous masses of coarse- 
grained rocks, which are undoubtedly closely related to the volcanics. At the present 
stage of the investigation it is, however, undecided just what this relation is. It is 
presumed that they represent great fissure flows, with which eruptive activity here 
began, and which were subsequently followed by the extrusion of the thinner lavas and 
ejection of the lapilli and ashes forming our present breccia and tuff deposits. Their 
general distribution is shown on the accompanying sketch map, PI. I. 
