40 ALASKA GEOLOGY 



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- 

 morphic 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 CO 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- 



