penceb.] TREADWELL ORE DEPOSITS, DOUGLAS ISLAND. 85 
)evond comparison with the effects directly or indirectly attributable 
;o a pair of narrow dikes of this sort. It is now believed that they 
lave no connection with the formation of the ore. 
Other basaltic dikes occurring in Gold Creek, near Juneau, are 
•egarded as practically of the same age as those on Douglas Island, 
md these are also unmistakably younger than the gold-bearing quartz 
reins of that neighborhood. 
ORIGIN OF THE FRACTURES. 
Upon the fracturing of the Treadwell dikes their impregnation with 
^old-bearing sulphides is evidently dependent. The systematic arrange- 
ment of the reticulating veinlets in two main sets standing at right 
ingles to each other and dipping in opposite directions led Becker to 
he conclusion that the fractures had been produced through com- 
)ressive shearing stresses. He suggested that these stresses were 
:aused by nearly tangential forces acting in a direction normal to the 
lommon strike of the two sets of fractures, which is also approxi- 
nately the strike of the country rocks." The fact that the fractures 
,re due to compressive thrust need not be questioned, since the theory 
>f the subject has been so ably developed and so fully corroborated 
>y experiment. 6 Some doubt arises, however, as to the direction in 
drich the forces may have been applied, because the geologic history 
>f the .general region since the diorite intrusions seems to indicate 
hat no widespread lateral compression has taken place. If tangential 
hortening has been going on, evidences of the fact, independent of 
he fracturing, has not yet appeared. On the other hand, a study of 
he wide physiographic features of this portion of North America has 
hown that a succession of continental uplifts has taken place since 
he period of the diorite invasion, and it seems necessary to suppose 
hat such radial movements would tend rather toward areal dilation 
han toward contraction, as in the opposite case of tangential com- 
>ression. c 
It is suggested that the general Assuring throughout the Juneau 
listrict may have been caused by gravitative adjustment in the 
ock masses, tending to restore internal equilibrium disturbed during 
he uplifts which are known to have taken place. The rocks of the 
listrict consist of alternating beds of greatly varying physical char- 
Icter, and they possess an eminent cleavage structure parallel with 
pe stratification. Under stress such rocks would }deld more readily 
(long the preexisting structure planes than in other directions. That 
Ihis old structure has, in fact, taken up most of the internal move- 
nent during the later deformation of the rocks is evident from the 
a Becker, G. F„ op. cit., p. 67. 
b Becker, G. F., Finite homogeneous strain: Bull. Geol. Soc. America, vol. 4, 1893, p. 13. Daubree, 
tudes Synthetique de Geologie Experimentale, p. 316. 
jcSpencer, A. C, The Pacific mountain system in British Columbia and Alaska: Bull. Geol. Soc. 
merica, vol. 14, 1903, pp. 117-132. 
