84 ALASKAN MINERAL RESOURCES IN 1904. [bull. 259. 
The alteration of the Tread well diorite is regarded as a phenomenon 
which accompanied the formation of the veinlets which intersect the 
jock, and the metasomatic action is attributed to the same solutions 
as those which deposited the quartz and calcite. The minerals last 
named appear to have been for the most part introduced, but the albite 
is believed to have been formed entirely, or nearly so, from the pre- 
vious minerals of the diorite, because it is not found in the larger vein- 
tillings. It is commonly observed that where both calcite and quartz 
are present in the fractures, the former usually occurs next to the 
walls, and it always permeates the rock to a greater or less extent. 
According to Lindgren, alteration of the sort here described has not 
been previously recorded, for though albite occurs as a vein mineral 
in California, it has not been detected among the metasomatic miner- 
als in the wall rocks of veins. a In this connection, however, refer- 
ence should be made to pseudomorphs of albite after adularia from 
St. Gotthard. These are described by Bischoff, 6 who gives an extended 
discussion of the probable chemical reactions involved, and suggests 
the competence of waters containing sodium chloride to effect the 
observed replacement of potash feldspar by soda feldspar. 
The occurrence of values in the wall rock to such an extent as is 
observed in the Treadwell ores is also somewhat unusual, though not 
unique. 
ROLE OF THE BASALT DIKES. 
In his discussion of the genesis of the Treadwell-Mexican ores, Doctor 
Becker leaves some doubt as to the importance which he desired to 
assign to the basalt dikes as mineralizers. He first says that the gen- 
esis of the ores is probably connected with the dikes, but afterwards 
.suggests the relative unimportance of their influence/ 
In the Treadwell and Seven Hundred Foot mines, two narrow dikes 
of the basalt are observed in a zone of sheeting, which is undoubtedly 
later than most of the veinlets in the ore mass. A small amount of 
calcite is found along their selvages, but they contain little or no 
pyrite. Upon the west or hanging- wall side of the dikes the ore is 
somewhat richer than it is between and beneath them, but it seems that 
this variation in gold tenor can not be attributed to the dikes as miner- 
alizers, because the rock between them is not enriched, as might be 
expected had they been an actual source of gold. Possibly a rear- 
rangement of values by relatively recent circulation has been goinj 
on, and the course of the currents may well have been controlled by 
the zone of sheeting in which the dikes occur, but secondary migra- 
tion of this sort must be distinguished from the original mineraliza- 
tion, the extensive results of which in the neighborhood are entire b 
a Lindgren, W., Metasomatic processes in fissure veins: Trans. Am. Inst. Min. Eng., vol. 30, p. 533. 
?>Chern. Geol., vol. 2, pp. 409-411. 
<* Eighteenth Ann. Rept. U. S. Geol. Survey, pt. 3, 1898, p. 69. 
