28 KA1RRAN KS AND RAMPART QUADRANGLES. 
The greenstones are partly intrusive and partly extrusive in the 
rocks in which they occur. Those in association with the limestone 
are, so far as has been observed, parallel to the structure, and further- 
more, some of them are altered basalts containing numerous amyg- 
dules filled with calcite. Diabasic intrusives cut the serpentine and 
in the Rampart region intrude the Rampart slates. The volcanic ac- 
tivity that resulted in the production of this material took place, prin- 
cipally at least, during the deposition of the rocks regarded as Silur- 
ian and Devonian. 
BASALT. 
A fresh olivine basalt occurs on Minook Creek about 1 mile above 
its mouth. On Hunter Creek, a short distance above its mouth, and 
apparently related to the basalt in their occurrence, are volcanic 
glasses containing basic feldspar phenocrysts. These volcanics are 
probably subsequent to the Kenai. A small mass of basalt was also 
observed in the Fairbanks region in the valley of Fairbanks Creek. 
METAMORPH< >SED R I [Y< >LITES. 
In the region south of Tanana River there are interbedded with 
the cherts and slates (Devonian?) large masses of augen gneisses 
which on microscopic examination proved to be altered rhyolites. 
These rocks range from a coarsely porphyritic gneiss with feldspars 
up to 2 inches in diameter to an evenly fine-grained sericite schist with 
no grains visible to the eye. The most common type is composed es- 
sentially of quartz and feldspar grains in a groundmass of quartz and 
feldspar largely sericitized. Remnants of original igneous textures 
are preserved, but the present structures and textures are due mostly 
to cataclastic action and recrystallization. 
SUMMARY. 
The greatest part of the area is formed of closely folded rocks strik- 
ing northeast and southwest, separable into two large groups, one in- 
cluding highly metamorphosed schists regarded as early Paleozoic or 
jDre-Paleozoic and the other being made up of rocks greatly variable 
in kind but characterized in general by a less degree of metamorphism 
and regarded as predominantly Silurian and Devonian. Igneous 
rocks are an essential constituent of the area. Some of them have 
been highly metamorphosed along with the sedimentary rocks that 
are now schists ; others are fresh granitic or more basic intrusives that 
were probably injected at the end of Mesozoic time ; and still others, 
which are present to a minor extent, are comparatively recent ex- 
trusives. Igneous rocks occur in all the areas that produce gold, but 
arc not confined to these areas, and the gold occurrences are probably 
to be referred partly, at least, to after-effects of intrusion. 
