ilo 
EAGLE RIVER SECTION. 169 
the matrix, with the same sort of devitrification, are besides entirely included 
in the quartz, and do not form any part of a projecting neck, as was plainly 
to be made out with a high power. The porphyritic feldspars in the section 
are mostly triclinic, and yield the angles for oligoclase. Crystals of augite 
as large as the quartzes occur, and with similar contours and embayments 
of the matrix to indicate their corrosion while the rock was still fluid. These 
augites are filled with the brown ferrite characterizing the augites of the 
previously described pebbles. 
The matrix of the conglomerate consists of the same material as the 
pebbles in a fine state, and more or less altered, being largely impregnated 
with calcite. Toward the base of the bed the matrix becomes relatively 
much more plentiful, until finally the conglomerate merges into a dark-red 
sandstone of varying coarseness of grain. There are included in this sand- 
stone some layers much darker than the rest, the usual red color being 
darkened by the admixture of some black particles. The thin section of 
this portion shows a quite loose aggregation of particles worn from the several 
kinds of acid rocks, including the matrix, as well as the porphyritic feld- 
spars and quartzes, which are readily recognizable as originally constituents 
of one of the porphyries. The peculiar secondary quartz so characteristic 
of the augite-syenites marks some particles as coming from that class of 
rocks. There is also a little infiltrated calcite and more or less earthy red 
iron oxide. So far, the rock is just like the predominating red sandstone of 
this bed, but there is in this section, in addition, an abundance of rounded 
particles of magnetite, to whose presence the dark color of the rock is 
undoubtedly due. 
Below this sandstone comes a succession of basic eruptive rocks, mostly 
fine-grained diabases, with their amygdaloidal portions, and occasional 
interstratified sandstone and conglomerate layers. Marvine has measured 
in detail a total thickness of 4,845 feet to the southernmost rock in the 
Phcenix mine, from the base of the sandstone layer just described. This 
thickness gives a width on the surface of 9,510 feet, the average dip being 
placed at 30°. Marvine has divided this thickness into four subordinate 
groups, which are described below in descending order. The descriptions 
are condensed from the macroscopic descriptions of Marvine’s detailed sec- 
