276 COPPER-BEARING ROCKS OF LAKE SUPERIOR. 
extent, in the more compact portions of the beds. The amygdules are rela- 
tively small and not very thickly strewn. They are frequently elongated 
in a common direction and ina very striking manner. The same amygda- 
loids which exhibit this elongation of the amygdules in a common direction 
show also other signs of having flowed as lava, such as flowage lines, ropy 
texture in a common direction, etc. The most interesting of these phenom- 
ena is, however, an appearance of stratification presented on a weathered 
surface, which is evidently directly connected with the viscous flow of the 
original lava. 
As to the thickness of the layers of this series at Duluth, it may be said 
that the thickness of the lower layers is exceedingly difficult to determine, 
the rock exposures showing the same material of great widths, and there 
being no amygdaloids. Evidently the thicknesses are very considerable, 
probably measured even by hundreds of feet. Higher up, however, the 
layers become plainly thinned, and alternate more rapidly, and in the upper 
third of the series amygdaloids and compact portions alternate with each 
other quite rapidly, as is well seen along the shore west of Chester Creek. 
An examination of a large number of sections of these rocks under 
the microscope showed them all to be closely the same, varying only in 
texture and fineness of grain. The main ingredients in all are plagioclase, 
augite and magnetite. ‘The triclinic feldspar occurs both in the ground- 
mass and as the chief porphyritic ingredient, the reddish crystals showing 
frequently and plainly the striation to the naked eye. In both, measure- 
ments give the low angles indicative of oligoclase. The porphyritic erys- 
tals are commonly filled with brownish particles of ferrite, and are nearly 
always more or less thoroughly dulled by alteration. The augite is only 
rarely fresh, generally showing more or less of a change to a greenish, non- 
polarizing, viriditic substance, with which change there is also connected the 
formation of much magnetite in small particles. It exists both as a filling 
to the spaces between the feldspars, and in little rounded granules, the latter 
phase being especially characteristic of the finer-grained kinds. Magnetite 
is present, however, not only as an alteration-product of the augite, but 
also occurs abundantly in all sections as an original constituent. Besides 
these main ingredients, there are also to be observed apatite, which occurs 
