254 The American Geologist. April, 189t 
Upon examination, the roek proved to belong to tlie type 
generally known as hornblende-basalt. It is a dark, heavy 
roek full of prominent crystals of hornblende from 4 to 25'""' 
in length. They are equally distributed, and appear to form 
10% of the v^'hole mass. A few small rusty and white spots 
are due to olivine and feldspar. The phenocrysts are in a dense 
dark groundmass, which, to the naked eye, looks like ordinary 
basalt. 
In the thin section, the deep brown basaltic hornblendes are 
seen to be rounded and embayed. They are anhedral instead 
of idiomorphic, as phenocrysts usuallj'- are, and each is en- 
closed in a darlv, magnetitic border, characteristic of horn- 
blende that has been corroded and partly resorbed by the 
magma. 
Although most of the hornblende is fresh, some of it has en- 
tirely disappeared, its place being occupied by a dark group 
of granular augite and magnetite. Some of the crystals show 
twinning parallel to the orthopinacoid. Although the twin- 
ning plane of inclined sections occasionally makes a considera- 
ble angle with the prismatic cleavage, as shown by Becke, this is 
not exceptional. Pleochroism is strong, varying from dark 
brown to light yellow. There are a few sharp crystals of au- 
gite with lateral pinacoids well developed and some are twinned 
parallel to one of these planes. Neither the augite, olivine^ 
nor feldspar show definite resorption phenomena. The olivine 
rarely has traces of crystallographic outlines. Much of it is 
changed to serpentine. 
The groundmass is almost holocrystalline, composed chiefly 
of plagioclase and augite, with much olivine and magnetite^ 
a few good crystals of apatite and a trace of clear base. The 
larger feldspar crystals are squarish and generally show nu- 
merous twinning bands. The more abundant smaller crystals 
of plagioclase are lath-shaped. Much of the yellowish green 
augite is in similar crystals with a large angle of extinction. 
Rarely it appears in irregular granules clamped between the 
feldspars, suggesting ophitic structure. The magnetite is 
often in sharp crystals. Some of it is probably titaniferous, 
judging from the analysis. 
The following chemical analysis of the Kosk creek horn- 
blende-basalt (No. 2) was made by L, G. Eakinsin the chem- 
ical laboratory of the U. S. Geological Survey. For compari- 
