FRINPLE.] GEOLOGIC SKETCH. 27 
the order in which they have been mentioned, without laying too 
much stress on their separation, sequence, or age. 
A formation composed of limestone, quartzite, shale, and chert 
beds, with closely associated tuffs, serpentine, and diabase, occurs 
above the older rocks, is much less metamorphosed, and is generally 
characterized by its green igneous content. This is the Rampart 
formation of Spurr. 
The massive conglomerate and shales of the Kenai form the young- 
est consolidated sediments observed. 
The Quaternary unconsolidated deposits include the bench gravels 
and stream deposits. 
The igneous rocks are both intrusive and extrusive. There are 
granular representatives which vary in composition from acidic 
granite to gabbro, and occur, so far as known, as intrusive masses in 
all the formations but the Kenai. Extrusives are associated with 
the rocks of the Rampart formation, and some limited areas of fresh 
olivine-basalt that were encountered probably represent flows of com- 
paratively recent origin. Dikes and sills are numerous and vary 
greatly in composition. 
1 PELLY GNEISS. 
Large areas of gneissoid rocks, provisionally assigned to the 
Archean, have been described, the most extensive one in the Yukon- 
Tanana country being found along the north side of the Tanana west 
from the boundary. This, as described by Brooks," consists of rocks 
which vary in character from comparatively massive granite to mica- 
schists, and may include some sediments. The rocks generally exhibit 
a gneissoid structure due to dynamic metamorphism. Areas of similar 
rock in the vicinity of the Fortymile were studied by Spurr'' and 
referred by him to the basal granite. Local areas of gneiss were 
observed in 1903, but the relations were not worked out in sufficient 
detail to justify definite conclusions as to their origin. It is believed, 
however, that they represent metamorphosed granitic intrusives, like 
those described by McConnell.' 
BIRCH CREEK SCHISTS. 
.The quartzite-schists have been described and named by Spun."' the 
Birch Creek formation. These rocks are most extensively developed 
"Brooks, Alfred EL, A reconnaissance in (lie White and Tanana river basins, Alaska, 
in 1X98: Twentieth Ann. Rept. U. S. Geol. Survey, pt. 7, 1900, pp. 460-470; A recon- 
naissance from Pyramid Ilarhoi to Eagle City, Alaska : Twenty-first Ann. Kept. I T . S. 
Geol. Survey, pt. 2, 1900, pp. 356-358. 
"Spurr, .1. E., Geology of the Yukon -old district : Eighteenth Ann. Kept. U. S. Geol. 
Survey, pt. 3, 1S9S. pp. 1. -,4-1 37. 
r < McConnell, K. G., Note on the so-called basal granite of the Yukon Valley: Am. 
Geologist, vol. 3<>, 1902, pp. 55-62. 
''Spurr, J. E., Geology of the Yukon gold district, Alaska: Eighteenth Ann. Rept. 
U. S. Geol. Survey, pt. :i, 1896, pp. 140 145. 
