74 ALASKAN MINERAL RESOURCES IN 1904. [bull. 259. 
of Douglas Island, the remaining portion being beneath Gastineau 
Channel. They are highly metamorphosed carbonaceous and calca- 
reous shales, of fairly uniform texture; their stratification is usually 
determinable from variations in color and from slight changes in the 
character of material, and in so far as observed the bedding and prin- 
cipal slaty cleavage are always in accord. 
The cleavage of the slates is regarded as having been produced 
before the diorite intrusions, the direction of which it largely controls. 
In this respect the secondary structure corresponds with that of the 
sedimentary rocks of the general region, all of which were tilted and 
metamorphosed before the diorites of the Coast Range were intruded. 
The slates are not altered b}^ contact metamorphism next to the intru- 
sive dikes of diorite. 
ALBITE-DIORITE. 
Classification of the Treadwell rock is somewhat difficult, because it 
has been impossible to procure entirely unaltered material. Doctor i 
Becker, who first studied it with care, gave it the designation ''sodium 
Bowlder 
Greenstone Clay 
Fig. 2.— Cross section through Alaska Treadwell mine and northern side of Douglas Island. 
syenite," to distinguish it from the ordinary syenites, which contain 
potassium as their alkali constituent. However, since the soda-f eldspar 1 
albite, which is the characteristic mineral of the rock, belongs to the j 
plagioclase series, and these feldspars are the distinguishing feature of j 
dioritic rocks, he suggested the alternative name " albite-diorite,'! 
which is here employed because it indicates the known relationship of 
the Treadwell rock with the diorite intrusives of the adjacent Coast 
Range. 
The rock varies in mineralogical composition from place to place,! 
but it is always very much changed from its original condition. Most! 
of it shows little or no ferromagnesian minerals, either because theyj 
were never present or because they have been decomposed and carried, 
away by the mineralizing solutions which have permeated the rock.J 
Specimens were collected, however, which contained hornblende in! 
apparently original prisms, and biotite is sometimes observed. Sec-I 
ondarily crystallized mica and green hornblende are somewhat com- 
mon, and with them a considerable amount of epidote is ordinarily' 
found. Feldspar is present in two conditions, original and secondary.! 
