4 ° 
ALASKA GEOLOGY 
(g) Dark reddish-grey fine-grained massive pyritous quartzite 
(217). It contains much red secondary biotite, and tremolite grains. 
Among the clastic quartz grains are many plagioclase grains, which 
are angular or rounded and perfectly fresh so that the twinning bands 
run sharply up to the boundary. Their shape and association make 
them seem fragmental, like the quartz grains, and yet they appear too 
fresh to have survived the influences that have produced so much biotite 
and hornblende and they are probably the results of the same meta- 
mo rphic agencies, which have here, as in many cases, produced 
rounded, pebble-like grains in a sandy matrix. 
( h ) Pure white, massive vein quartz . 
(z*) Greywacke-hornblende-schist. A metamorphosed grey wacke, 
containing grains of granite minerals and slate; the broad interstices 
filled up with tufts of actinolite needles forming a cement of horn- 
blende-schist for the whole. 
This is a very remarkable rock. The granite quartz grains are full 
of cavities with reddish refringent C 0 2 and moving bubbles. The 
feldspar and plagioclase grains are little changed, and yet the whole 
cementing mass is made up of minute delicately-tufted actinolite 
needles, which often radiate from the corners of the grains. The 
original cement may have been basaltic or have been the common cal¬ 
careous and ferruginous cement of a sedimentary rock. 
(/) A reddish-brown fine-grained micaceous quartzite . 
(k ) A dark grey cherty slate , with white spots that seem like re¬ 
mains of fossils. 
(/) A black, almost aphanitic, quartzite , full of minute interlacing 
quartz veins. 
(m) A black flinty pyritous slate, banded with thin white quartzite 
layers. 
This is an interesting series of metamorphic rocks. 
The preservation of all the clastic grains of a greywacke 
intact while the paste has changed into an amphibolite is 
remarkable, and the preservation of grains of plagioclase 
in ideal freshness in a rock full of secondary biotite and 
hornblende is also noticeable. 
The series seems to be due to contact metamorphism, 
and recalls many of the varieties of the crystalline rocks 
around Glacier Bay, where the limestones have been re- 
